Armor Repair only gives up to 20 armor, it will save your life by simply preventing you from being at 0 Armor. As long as you have any armor, you gain up to 30% damage reduction, depending on your armor level. Base Armor gives 0% damage reduction, Green gives 5%, Blue gives 20%, and Purple gives 30%. It's generally never worth the dosh to upgrade to Green unless you can also afford Blue, not even for the free armor you get when upgrading.

Passive proc chances are multiplicative, not additive. Which means that 3x Deadly Resupply doesn't add up to 99% proc chance (33%+33%+33%). Instead, the combined probability is calculated as 1−(1−0.33)^3, which gives a 70% chance to proc at least once. 2x Deadly Resupply would be 1−(1−0.33)^2 = 55% chance.

This also means that 3x Critical Strikes does not add up to a 4.5% proc chance. The tooltip is worded "Each shot has a 1.5% chance to deal 150% damage" rather than "to deal 150% more damage", meaning the actual probability is 1−(1−0.015)^3 = 0.0443% per shot to deal 50% more damage, making it one of the weakest passives for all mod slots.

As you increase difficulty up to Hell on Earth, you'll notice zeds move much faster. Normal running won't cut it, eventually you're going to need to learn some move tech. The easiest method is simply swapping to a knife, then there are other techniques like Diag-Dashing and SlideJumping. Thankfully every perk has a move speed skill, don't underestimate them.

Zeds always spawn near you. Shocking, right? More specifically, in multiplayer, they will always spawn near all players. If you're grouped up, it will force zeds to spawn from limited spawn points and then attack from more predictable routes. By funneling zeds down hallways, around corners, or through doors you can make each wave easier and faster for bunker strategies.

Try not to be off on your own. If you feel the need to be off protecting another doorway or flank... you are probably the reason zeds are spawning there. And definitely don't be that player who farms all of the kills while the rest of the party is grouped up waiting for the Zeds to come around the corner.

Ballistic damage is the most common, but could easily be considered the weakest. Force damage doesn't seem to stand out much with only one strength, but it has no weaknesses either. Being found on Explosive Weapons certainly helps, but other damage types are also effective.

Scrakes and Fleshpounds will be your biggest obstacles during waves, so bringing something to deal with them ASAP is going to be your highest priority, especially on HoE where they start showing up early! By the boss wave you should have enough dosh for two weapons to cover any potential weaknesses.
Weakness: Ballistic, Cut, Biological
Strength: Force, Electrical
Weakness: Force, Electrical
Strength: Ballistic, Cut, Biological
Weakness: Fire, Biological
Strength: Force, Acidic
Weakness: Force, Cold
Strength: Fire, Electrical
Weakness: Electrical, Acidic
Strength: Cold, Biological

Damage over Time: Bleed (Cut), Burn (Heat), Corrosion (Acidic), Toxic (Biological)
Hard CC/Interrupts: Confuse, Freeze, Knockdown, Panic, Shock, Stumble, Stun
Enfeeble is unique as the only debuffing effect without a DoT or interrupt, which means you're free to apply this debuff without worrying about overwriting other CC effects.

Weakpoints on Scrakes and Fleshpounds are either Armor or Zedtech. Exposed fleshy bits are vulnerable to Ballistic/Biological/Cut, but that doesn't mean all or any of the weakpoints are flesh.
Based on a combination of damage resistances (±15%) and weak zone multipliers (±10%), you can calculate how much each damage type will actually deal, depending on where you hit.

• Ballistic/Cut: Hollow Point Rounds, Kunai (throw/primary), Shuriken
  
• Ballistic/Force: Armor Piercing Rounds, Reactive Alloy Rounds
  
• Ballistic/Heat: Incendiary Rounds, Incendiary Shells, White Phosphorous Shells
  
• Ballistic/Cold: Cold Ammo, Samaritan (no ammo mod), Disciple (secondary)
  
• Ballistic/Electrical: Electric Ammo, Electric Rounds (Samaritan), Electric Slugs
  
• Ballistic/Acidic: Acidic Ammo
  
• Ballistic/Biological: Enfeebling Rounds (Pyre .357), Enfeebling Slugs, Poly-Synthetic Rounds
  
• Force/Soundwave: Explosive Rounds, Explosive Slugs, Overpressure Rounds, Frag Launcher (Knightsbridge), G33 (no ammo mod)
  
• Force/Heat: Incendiary Grenades, Vulcan TA (primary)
  
• Force/Acidic: Acid Grenades (DoT pool)
  
• Force/Biological: Enfeebling Smoke Grenades - G33, Biomorphic Projector (underbarrel)
  
• Cut/Electrical: Electrified Blade (Kiba), Dual Katanas (secondary)
  
• Cut/Force: Sai Blade Kunai (melee/secondary)
  
• Biological/Electrical: LF-85 (secondary)

• Allied Zed Time (multiplayer)
  
• Armor Repair (especially useful for melee coatings)
  
• Armored Allies (kill reliant, multiplayer+)
  
• Boss Bash Team-Up (multiplayer bosses++)
  
• Boss Basher (bosses only)
  
• Deadly Resupply (kill reliant, not good for weapons that use multiple ammo per shot)
  
• Kill Shield (kill reliant)
  
• Killshot Cure (kill reliant, multiplayer+)
  
• Melee Master (melee only)
  
• Stumblesome (all-purpose)

Passives like Armored Allies, Boss Basher Team-Up, and Killshot Cure may not directly affect your character, but they're the most valuable passives available for multiplayer when used effectively.

• Ammo Efficiency
  
• Blood Transfusion
  
• Bounce Back
  
• Combat Cure
  
• Coordinated Carnage
  
• Critical Strikes
  
• Deadlier Zed Time
  
• Last Stand
  
• Long-Distance Operator
  
• Toxicity

Whether because of their atrocious activation conditions, abysmal probabilities to proc, or otherwise inconceivably insignificant impact, these passives will likely need a bit of balancing before they become viable.
For most slots there is a pool of 15 possible passives to roll from, and each passive can roll in up to 3 slots at most. Some passives line up just right and can be stacked without overlap for 3+3 like Deadly Resupply / Kill Shield.

Ninja Weapons are a bit different, just like their parts and mods. Most mod slots only have 8 possible passives, aside from Sights with 12. Some passives can only roll once, like Armor Repair on Coatings, but a few passives can still roll 3x, such as Melee Master and Kill Shield.

Gadget: Hellion (Acidic)
An autonomous drone that targets enemies with acidic projectiles.
Throwable: HE Grenade (Force)
A conventional military grade grenade with a short timer.

Passives: Fire Sale, Preferred Customer, Booster Shot, Time and Again, A Real Rager
Throwable: Think Fast, +1 Cap, Corrosion Explosion, +1 Cap, Scavenger
Gadget: Market Panic, Extended Coverage, Slow Dance, Quick Return, Smog Warning

Get Loaded can be excellent if you plan to get a secondary weapon to swap between, especially since you have Preferred Customer to allow early access to stronger weapons. Cluster Bombs are a powerful alternative to Corrosion Explosion if you prefer immediate results.

Ammo: Armor Piercing / Overpressure Rounds
Barrel: Suppressor
Underbarrel: Enhanced Foregrip
Sight: Reflex Sight
Magazine: Extended Magazine (optional)
Internal: Heavy Frame Receiver
I'd recommend starting with your Ammunition of choice and the Heavy Frame Receiver first since they're the only mods that actually increase your damage. Add on the Suppressor, Reflex Sight, and Foregrip as your starting budget increases.
Ammo: Armor Piercing / Overpressure Rounds
Barrel: Suppressor
Underbarrel: Enhanced Foregrip
Sight: Reflex Sight
Magazine: Extended Mag
Internal: Heavy Frame Receiver

Ammo: Armor Piercing / Overpressure Rounds
Barrel: Suppressor
Underbarrel: Laser Sight
Sight: Reflex Sight
Magazine: Lightweight Mag (optional)
Internal: Heavy Frame Receiver
Don't be fooled by the Drum Mag, it's not worth the dosh.
Ammo: Reactive Alloy Rounds
Barrel: Mag-Channel Accelerator
Underbarrel: Frag Launcher
Sight: Reflex Sight
Magazine: Powered Box Mag
Internal: Heavy Frame Receiver
Poly-Synthetic Rounds (Ballistic/Bio) do great against smaller zeds, but deal even less against Fleshpounds. Luckily, the Frag Launcher brings exactly what you need in the form of Force/Soundwave damage.
Ammo: Explosive Rounds
Barrel: Suppressor
Underbarrel: Laser Sight (Pistol)
Sight: Reflex Sight (Pistol)
Magazine: Extended Mag
Internal: Multi-Round Burst Fire Mode (optional)
AP and Electric rounds work fine too

Gadget: Wildfire (Heat)
A back-worn thermal energy device that releases an explosive ring of fire.
Throwable: Molotov Cocktail (Heat)
A breakable incendiary weapon that ignites an area and all enemies within it.

Passives: Firewall, Focus Fire, Fuel to Burn, Red Zone, Extinguisher
Throwable: Cocktail Party, +1 Cap, Extended Edition, +1 Cap, Asphyxiate
Gadget: Heat Therapy, Speedy Exit, In the Public Eye, Hot Streak, Flammable Material

Ammo: White Phosphorous Flares
Barrel: Magnesium Accelerator
Underbarrel: Empty
Sight: Reflex Sight
Magazine: Extended Mag
Internal: Heavy Frame Receiver
Ammo, Barrel, and Receiver should be your first priorities.
Ammo: White Phosphorous Shells
Barrel: Compensator
Underbarrel: Enhanced Foregrip
Sight: Reflex Sight
Magazine: Extended Mag
Internal: Affliction Amplifier

Ammo: Tar Fuel
Barrel: Wide-Angle Nozzle
Underbarrel: Incendiary Launcher
Magazine: Extended Fuel Tank
Internal: Slowed Tick Receiver

Ammo: Supercharged Plasma
Barrel: Plasma Focuser
Magazine: Pressurized Tank
Internal: Pneumatic Plasma Cooler
This plasma cannon is easily one of the best T4 weapons thanks to the combination of Heat and Force damage along with an explosive impact capable of hitting multiple weakpoints or blowing up entire hordes of zeds. Ultra Octane Fuel Tank provides a bit more damage at the cost of ammo efficiency, but Pressurized Tank will provide a sizable amount of increased damage per magazine.
Ammo: Electric Ammo / Enfeebling Rounds
Barrel: Suppressor
Underbarrel: Laser Sight(Pistol)
Sight: Reflex Sight
Magazine: Suppressed Cyclinder
Internal: Heavy-Action Hammer
This underrated sidearm packs a punch with higher damage than Sharpshooter's Disciple, only without the explosive rounds. You can swap for the Upgraded Speed Loader if you want more reload speed and don't mind the recoil.

Gadget: Sonic Storm (Sound Wave)
An augmented power suit with two turrets that obliterate enemies with sonic projectiles.
Throwable: Bouncing Betty (Ballistic)
A trigger-pressured mine that detonates to create a lethal circle of shrapnel.

Passives: Tool up, Major Damage, Bleed for Speed, Full Blast, Rally Beacon
Throwable: Chaos Theory, +1 Cap, Dismember, +1 Cap, Heated Exchange
Gadget: Fortify, High Frequency, Juggernaut, Reforge, Reverb Pedal

Taking Scatter Plot (20) instead of Full Blast will greatly increase the damage you deal with the ConTek CSG, Bonebreaker, and Dragon's Breath for any non-slug ammo, but as there currently aren't any T3 or T4 weapons that use pellets I recommend speccing for Explosives instead. Gadget skills aim to take advantage of Reverb Pedal's flat AoE damage along with High Frequency for more pulses. Tool Up and Reforge provide amazing synergy with Ammo Bags, letting you exceed the normal wave limit by dropping multiple bags each wave.

Ammo: Explosive Slugs
Barrel: Shotgun Suppressor
Underbarrel: Laser Sight
Sight: Reflex Sight
Magazine: Lightweight Mag
Internal: Empty
Easily the best T1 weapon of any perk, so much so that it's the only T1 weapon I ever bother buying on other perks. Sporting great base damage and a point-and-click playstyle thanks to Explosive Slugs, this shotty will put a hole through just about anything short of Scrakes and Fleshpounds. This ability to one-shot most targets also makes it an ideal candidate for Deadly Resupply.
Ammo: Explosive Slugs
Barrel: Shotgun Suppressor
Underbarrel: Laser Sight
Sight: Reflex Sight
Magazine: Drum Mag
Internal: Heavy Frame Receiver
Bonebreaker feels fine with explosive slugs, but Scatter Plot and Incendiary Shells really make this weapon shine. Unfortunately there aren't any T3/4 weapons that use pellets (yet) so I don't recommend Scatter Plot.
Ammo: Incendiary Grenades
Barrel: Extended Barrel
Sight: Red Dot Sight (optional)
Magazine: Eight Round Cylinder
Internal: Affliction Amplifier
All of the ammo types can be useful, although some ammo types turn it into a support weapon rather than damaging. Acid and Incendiary grenades both leave pools on the ground, so you'll end up using them much like the Smoke Grenade variants by aiming at the ground.
Ammo: Fast Focus Array
Underbarrel: Enhanced Foregrip
Sight: Reflex Sight
Magazine: Extended Canister
Internal: Ultrasonic Plasma Cooler
Ifrit has great stats and a solid ammo pool. The primary fire plasma cutter beam couldn't be easier to use, just point at heads. The secondary fire is an shotgun-like electric pulse that helps quickly take down Fleshpounds with chest or headshots. Solid boss weapon against Impaler and Queen Crawler.
Ammo: Electric Ammo
Barrel: Compensator
Underbarrel: Laser Sight (Pistol)
Sight: Reflex Sight (Pistol)
Magazine: Extended Mag
Internal: Heavy Frame Receiver
Using the alt-fire burst mode helps keep the recoil and spread under control while retaining a similar fire rate to full auto.

Gadget: Death's Hand (Ballistic/Force)
A forearm-mounted launcher that fires a homing arrow that seeks and eliminates enemies.
Throwable: Cryo-Grenade (Cold)
A supercooled liquid helium grenade that freezes enemies in their tracks.

Passives: Hunter, Easy Prey, Hardshell, Flight Response, Big Game Hunter
Throwable: Freezerburn, +1 Cap, Blizzard, +1 Cap, Polar Vortex
Gadget: Bloodsucker, Sting Operation, Killer Bee, Target Run, Frosted Tips

Ammo: Explosive Rounds
Barrel: Suppressor
Underbarrel: Enhanced Foregrip
Sight: Reflex Sight
Magazine: Extended Mag
Internal: Air-tight Receiver
While you can afford to start with the Artemis, the HR-10 remains the better choice thanks to explosive rounds and higher base damage, letting you point-and-click your way through early waves.
Ammo: Cold Ammo
Barrel: Suppressor
Underbarrel: Enhanced Foregrip
Sight: Reflex Sight
Magazine: Extended Mag
Internal: Empty
Neither receiver option is worth the dosh. While you can get Explosive Rounds on the Artemis, I've found Cold Ammo to be easy to play with by freezing to line up easy headshots. Be sure to swap from automatic mode to semi-auto for faster fire rate.
Ammo: Armor Piercing Rounds
Barrel: Compensator
Underbarrel: Enhanced Foregrip
Sight: Sniper Scope
Magazine: Extended Mag
Internal: Hyper Velocity Receiver

Ammo: Reactive Alloy Rounds / Poly-Synthetic Rounds
Barrel: Heat Disperser
Underbarrel: Enhanced Foregrip
Sight: Reflex Sight
Magazine: Extended Mag
Internal: Tungsten Receiver
Hard-hitting railgun that trades Cold damage for Force damage with Reactive Alloy Rounds (or Biological with Poly-Synthetic Rounds), but still applies Freeze thanks to the Heat Disperser.
Ammo: Explosive Rounds
Barrel: Suppressor
Underbarrel: Laser Sight (Pistol)
Sight: Reflex Sight (Pistol)
Magazine: Suppressed Cylinder
Internal: Tungsten Receiver
A hand cannon with Explosive Rounds that also has a secondary Cold shotgun underbarrel that can Freeze your enemies. A fully modded Disciple is strong enough to use as a Starter weapon in place of a Primary weapon.

Gadget: Hebi-Ken (Electrical/Force)
A grappling hook that allows Nakata to pull himself towards enemies to deal devastating damage.
Throwable: Caltrop Canister (Electrical)
A throwable container that bursts outward with multiple electrifying caltrops.

Passives: Kenko, Roaring Tiger, Demon Stance, Chi kai ri, Unyielding
Throwable: Swift Current, +1 Cap, Thunderclap, +1 Cap, Inazuma
Gadget: Shinobi Shozoku, Dead Calm, Red Shadow, Lightning Incarnate, Leopard Fist

Block timing is currently inconsistent in multiplayer and thus not worth using most parry skills. Substitute Roaring Tiger for Kaishakunin if you're not using a melee weapon/heavy attacks. Punishing Chain can also aid your survivability with more Hebi-Ken charges, each granting 8 armor and 5s of Red Shadow uptime.

Coating: Stunning Coating
Grip: Reinforced Grip
Blade: Electrified Blade
Guard: Ornate Guard
Pommel: Balanced Pommel
The Electrified Blade is what really helps Kiba hold its own thanks to Fleshpounds vulnerability to Electrical damage. Although you can only get Armor Repair on your coating, and it only gives up to 20 armor, it will save your life by simply preventing you from being at 0 Armor. As long as you have any armor, you gain up to 30% damage reduction.
Coating: Enfeebling Venom
Grip: Textured Grip
Blade: Sai Blade
Pommel Ring: Weighted Pommel Ring
Ammo Pouch: Magnetic Ammo Pouch
Kunai are insanely strong as a melee weapon, and for some reason the stats appear to be highest for throwing when using the melee grip and blade. Bash is incredibly powerful, especially with a few Melee Master/Zed Time Rampage passives. Note that swapping to throw is significantly more effective against Scrakes than melee damage.
Sight: Laser Sight (optional)
Arrow: Exploding Arrows
Frame: Flexible Frame
Quiver: Large Quiver
String: Reinforced String
Exploding Arrows are expensive to restock, so it's better and cheaper to invest in Ammo Bags. Burrowing Arrows are also decent and most importantly they're retrievable.
Coating: Stunning Coating
Grip: Padded Grip
Blade: Tempered Blade
Guard: Heavy Guard
Pommel: Charged Pommel
This setup focuses on Block Power and Block Stamina, enabling you to do some truly nutty things, like facetanking bosses, even blocking Queen's P2 instakill charge. Don't underestimate how much damage your heavy attacks will do, especially with their secondary mode. Armor Repair Coating is your best choice for all the same reasons it's amazing on Kiba.
Coating: Stunning Coating
Blade: Triple Blades
Ammo Pouch: Magnetic Ammo Pouch
The fact that these have a base cost of 1200 and only have 3 mod slots is absolutely criminal. Sure, they give you as bit of move speed compared to other sidearms, but at what cost? 1200.

Gadget: Sanctum (Biological)
A barrier projector that heals all allies and damages all enemies within its radius
Throwable: Biotic Grenade (Biological)
A gas grenade whose biotic cloud fortifies friends and debilitates foes.

Passives: Doctor's Bag, Pill Mill, Symbiosis, Social Distancing, Chemical Reaction
Throwable: Cull the Weak, +1 Cap, Anesthetist, +1 Cap, Lingering Cloud
Gadget: Power Surge, Burn Unit, Field Hospital, Storm Shelter, Double Dose

Social Distancing is arguably Medic's strongest skill and their entire reason for existing in multiplayer. Heal and Deal can be used solo by running Symbiosis to proc self healing. The extra gadget duration provided by Storm Shelter allows extra time between Double Dose since you are able to activate Sanctum again any time before the first one expires.

Passives: Doctor's Bag, Pill Mill, Painkiller, Social Distancing, Heal and Deal
Throwable: Gas Expansion, +1 Cap, Anesthetist, +1 Cap, Lingering Cloud
Gadget: Power Surge, Outreach Center, Field Hospital, Energy Cycle, Resuscitator

Urgent Care can also be incredibly potent as your darts can be blocked by enemies or even allies. Stronger Prescription seems to be bugged atm, but more ammo still helps more to generate darts on your LF-85.

Ammo: Armor Piercing Rounds
Barrel: Suppressor
Underbarrel: Confusion Dart Attachment (optional)
Sight: Reflex Sight
Magazine: Extended Mag
Internal: Heavy Frame Receiver

Ammo: Efficient Battery Chemistry
Barrel: Confusion Additive Injector
Underbarrel: Healing Grenade Launcher (optional)
Magazine: Extended Battery
Internal: Slowed Tick Receiver
No matter how much I want this thing to work, the beam often seems to have a mind of its own.
Ammo: Armor Piercing Rounds
Barrel: Suppressed Barrel
Underbarrel: Healing Grenade Launcher
Sight: Reflex Sight
Magazine: Extended Mag
Internal: Heavy Frame Receiver
Abysmal T3 weapon. Healing Darts having to compete for the Ammo mod slot hurts for all the same reasons as the VS-11, only with a base cost of 3300 instead of 1400.
Ammo: Efficient Battery Formulation
Barrel: Cold Fusion Converter
Underbarrel: Enhanced Foregrip
Sight: Reflex Sight
Magazine: Primary Battery Booster
Internal: Undervolted Receiver
Your healing darts will only heal 4hp, and with only 15 darts dumping your entire mag will only heal 60hp, the same as 3 Rapid-Release Darts from your Stim Pistol. This is is just enough for clutch Social Distance revives, but don't expect too much else. I tend to use this build more for Sharpshooter's boosted Cold damage than Medic.
Ammo: Biomorphic Boosted Battery
Barrel: Cold Fusion Converter
Underbarrel: Enhanced Foregrip
Sight: Reflex Sight
Magazine: Primary Battery Booster
Internal: Undervolted Receiver
Healing darts do the same base 3 healing, but with the Biomorphic Boosted Battery and Undervolted Receiver (+35%) they are increased to 12 + 2.7/s for 3s, or ~20 healing per dart. While they aren't limited like the VS-11 or MP6, they don't regen over time like your Stim Pistol either. Using Primary Battery Booster here lets you primary fire more to regen darts faster, synergizing with Heal and Deal to increase both damage and healing.
Ammo: Rapid-Release Darts
Barrel: Suppressor (optional)
Underbarrel: Empty
Sight: Reflex Sight (Pistol) (optional)
Magazine: Lightweight Mag (optional)
Internal: Dart Recharger Kit
The Stim Pistol is one of the best and most versatile sidearms available, allowing any class to heal with nothing but a Dart Recharger and either of the Healing Dart mods. Base Darts heal for 3 HP.
Instant Heal Darts: +14 healing = 17 total Healing per Dart
Rapid-Release Darts: +6 hp/s for 3s = 21 total Healing per Dart
While Rapid-Release Darts do heal for more overall, three Instant Heal Darts are just barely enough to pull off a rez (102hp) with the Social Distance skill. However, any skill that buffs you or your allies based on when you heal is extended by the duration of the Heal-Over-Time effect.

If you've made it this far, thanks for taking the time to read my guide. I've appreciated all the support! Shout out to the PhaseShifter Community!

Now that Hotfix 4 has dropped and the Rearmament Update has been officially announced for Halloween, it means I'll be getting back to work adding more content. If you have any requests for features or additions to the guide, drop it in the comments below.

Special thanks to Tantrum9, creator of the killer KF3 companion site with an interactive skill builder, ZED-Kit[zed-kit.com]for the skill tree templates.