Some terms you should understand and memorize.

High Value Target
[Fleshpounds, Quarter Pounds, Scrakes]

Tier # of weapon. Higher is usually better.
You start with a Tier 1 weapon. T5 weapons are usually upgraded T4 weapons, however some weapons in the Trader start at Tier 5 (eg. M99)

Fleshpound

Quarter Pound

Scrake

Hell on Earth difficulty.
The highest vanilla difficulty in-game.

A series of actions executed by the player in order to safely and quickly kill a zed, usually HVT.

Rate of Fire.
How fast a weapon fires.

Damage Per Second. Not to be confused with Damage per Shot.

Damage over time.
Applicable to every Firebug weapon and poison (from Medic weapon darts and Zedative).

Piece of ♥♥♥♥.

running away from Medic Grenades
They are there to heal you. If you stand in them, you get healed. Isn't life amazing? In all seriousness though, make sure you stand in the blue gas as they apply continuous buffs to your person, and constant heal. I see too many people running away or ignoring the healing grenade gas for reasons that elude me. Stay in them. Make your Medic's life a little bit easier, and yours literally longer.

standing while King Fleshpound shoots his chest beam
Because it will kill you? I don't have to spell this out, right? It's surprising how many people don't know that crouching will let you dodge the beam. Unless you're on higher ground, in which case, good luck.

taking dead people's weapons and selling them at the Trader
This is just plain rude. Common courtesy is to return their stuff to them, because they might not have enough Dosh to replace it if they lose it. Killing Floor 2 is a team effort, not a race to see who can hoard the most Dosh or kills.

killing trash zeds in front of a Commando during Zed Time
Commando has a unique ability to extend Zed Time duration (reset it, technically) every time they kill a zed during the existing Zed Time. By taking trash zeds in front of them, you are essentially depriving them (and by extension the whole team) of Zed Time extensions, which are extremely important in HoE games.

micspamming over voice-chat
Obvious. Annoying and prevents players from hearing audio cues or important callouts. To the people who do this: Stop.

healing yourself when you're next to nearly-dead teammates
Apart from the fact that they need the syringe heal more than you do (unless you also happen to be almost dead), healing another player charges the syringe twice as fast than if you had healed yourself. Also you get a small amount of Dosh. Always share your needle.

buying armor on Wave 2
Armor should be one of the player's last priorities, the other being buying ammo for your 9mm Pistol. While it is true that armor can protect you from otherwise fatal hits, on lower waves, they will not occur as often as on later waves. By buying armor on lower waves, you are wasting the 300 dosh you could have used to buy more-needed ammunition or a better weapon, as trash/medium zeds will slowly whittle away your irrecoverable armor. Armor is temporary, but weapons stay forever.

buying the Pulverizer
Compared to the rest of Berserker's T3-T4 weapons, this is a rather underwhelming weapon for taking out HVT. As the explosive damage on the Pulverizer only does body damage, it takes quite a while to kill a FP or SC. Good crowd control, but has limited ammo and reload, and the Katana/Zweihander is far superior for trash. You are better off with the Zweihander, Static Strikers, or Bone Crusher.

buying armor on lower waves
You are literally going to lose it in the first minute of the wave. As Berserker is a melee-oriented perk, they will be taking lots of hits, and any bought armor will quickly fall to pieces. Usually, buying armor is not a bad idea, as it soaks up what could otherwise be fatal hits, but to a Berseker armor is of the LOWEST concern.

buying the L85 Bullpup Rifle
It's an under-powered POS. Does the same damage per bullet as your starter AR-15 Varmint Rifle, with only 30 extra bullets in reserve and a slightly higher RoF. Save yourself the trouble of wasting dosh and skip to the AK-12, which is an excellent all-purpose Commando rifle.

buying the Stoner 63A LMG
It's an expensive under-powered POS. This is a tier 4 weapon that has little to no HVT takedown capability, being almost exclusively suited for trash and medium killing. Does the same damage per bullet as your starter AR-15 Varmint Rifle, although with a drastically increased ammo reserve count and RoF. Just buy the HMTech-401 Medic Rifle or the FN-FAL ACOG, they will both serve you better in the end. Unless you are trying to live your Heavy Gunner perk fantasies out, in which case, go nuts.

welding doors around an area for no reason
Welding doors around a spot for the sake of welding doors is a sure-fire way to get yourself and your teammates trapped and killed. Realize that zeds may pile up behind welded doors and bust through, making for a nice player-slaughtering jamboree. There are many strategic uses for welding doors, but make sure to communicate with your teammates first before you do so. If you are welding doors for Support XP, have the courtesy to un-weld them after.

squeezing off shots with your HZ-12 to kill that siren across the map
Shotguns are not known for their accuracy over long ranges. Save yourself the ammo and effort. Let the zeds come closer to you, where you are at your most effective, before you shoot them in their ugly faces. And chances are, there will already be a Sharpshooter or Commando lining up a shot, so it's better to let them take the longer shot than to accidentally stumble their target.

standing so close to the Trader when you are buying weapons
This may come off more as a nit-picky thing than anything else, but when you are standing next to the Trader, nobody can retrieve ammo from you. Sometimes, a few dosh saved up for a new weapon can mean the difference between life and death. Don't rob your fellow teammates of their opportunity to purchase a weapon with their dosh they saved not buying extra ammo. Instead, jump backwards in the buy area, and while you are flying backwards, use the Trader mid-air. That way your teammates will be able to retrieve ammo from you easily.

using the Hemogoblin
It is an under-powered POS. Too heavy to take with the HMTech-401 Medic Rifle, the debuff speed is unsubstantial, the dart recharge rate is on-par with the HMTech-301 Medic Shotgun, and the DoT will probably rage HVT. You are better off taking weapons that heal+buff your teammates rather than damage+debuff your enemies.

taking all right-side skills
If you are playing the role of Field Medic, your duty is to assist by buffing+healing your teammates and killing lesser zeds. Not to make yourself a worse Survivalist with a faster healing syringe. If you are going to be a combat medic, play Survivalist instead -- they get 25% global damage increase, faster syringe recharge (but not as much as Medic's) and healing grenades (if you take the right skill).

Stop:

buying the HMTech-201 Medic SMG

Why:

It is an under-powered POS. The dart recharge rate is barely a smidge above the HMTech-101 Medic Pistol, and does roughly the same damage per shot.

Note: The HMTech-201 Medic SMG has received many buffs since the writing of this guide. It is still by no means a "good weapon", but now it is just barely powerful enough to justify the purchase on a low wave, instead of being absolutely useless. I still recommend you skip the SMG.

firing the M79 at Scrakes
Scrakes have a MASSIVE explosion resistance (x0.4 incoming damage), combined with the M79's single-shot mechanism, means that you will end up doing nothing but making the Scrake mad. The M79 is better reserved for crowd control of trash zeds, and if in a dire emergency, Fleshpounds (x2.33 incoming damage). Although it is not recommended to shoot at a calm Fleshpound either.

Note: Scrakes have a unique weakness to RPG-7 impact shots (x4 incoming damage), making a headshot with the RPG-7 rocket extremely powerful. On lower difficulties, this rocket can immediately decapacitate the Scrake with one shot, however on HoE this will usually require a .500 Magnum headshot beforehand.

standing right next to the Trader during trader time
Same reason as Support, teammates will be unable to retrieve grenades. Jump backwards out of the buy zone while using it mid-air to allow your teammates to take grenades.

spam-detonating C4 and expecting to kill Fleshpounds by doing so
Every single C4 explosion reduces the next C4 explosion damage by 25% if detonated within 3 seconds. This stacks over to successive C4s, as long as they are all detonated within under 3 seconds of each other. This is to prevent C4 spam (like Pipebombs from KF1). Instead, it is better to combo C4 explosions in between other weapons in order to pass the delay, such as C4 + RPG + Dynamite + C4.

setting HVT on fire
By setting HVT on fire, you are activating "Fire Panic", which is a type of affliction that makes zeds run and flail around. Because they are running around, it makes a harder target to hit for HVT-suited perks such as Gunslinger or Sharpshooter. Not to mention that fire does almost nothing to HVT, except make them mad, and you do not want to make them mad without a way to kill them. Save your team some grief and don't set them on fire, no matter how strong the urge is.

using the Husk Cannon
It is an expensive and under-powered POS. For real, this time. This weapon is so hilariously terrible that the thought of someone using this to kill a Fleshpound makes me sick. It can only hit the torso hitbox, it can inflict massive self damage (either by explosion or by fire, or both), the actual damage is absolutely horrible, and it does not have any impact damage whatsoever. It's a steaming pile of hot garbage. I would rather you use literally anything else than this. Please.

Note: Since the Cyperpunk Beta release and the introduction of the Helios Rifle, which costs exactly the same amount of dosh (1500), I urge you to use that instead. It's better in every way possible. And it can actually take down HVT.

buying the Spitfire/Dual Spitfires
The Spitfires are Gunslinger's weakest weapon selection. 40 projectile damage vs. the starter 1858 Revolvers' 50 projectile damage (both weapons have the same RoF). Also the only Gunslinger weapon that applies DoT and Fire Panic, as well as having gravity-affected projectiles. And they're, like, really bright. A little bit distracting. A waste of Dosh, as these are T2 weapons.

buying the Spitfire
Same reason outlined for the Dual Spitfires in Gunslinger's section. It is weak and underwhelming for its crossperk. If you are going Rail/M99-Sharp, and trash is your concern, either buy the much cheaper (50 Dosh) and capable 1858 Revolver or save for a .500 Magnum.

taking Battering Ram
This skill is arguably one of the least useful skills in the game, as you only have a mere few seconds (assuming there is no Commando to extend) to knock down a few zeds. And then, you have to decap every single zed you've knocked down in all directions individually. Knocking everything down in front of you sounds good on paper, but the reality is that it's a pain to use. In contrast, Rapid Assault is much more favorable as it lets you tear through crowds and Scrakes without wasting any ammunition, instead of just knocking them down.

Note: Yes, I know it is fun to see the boss being knocked out of the map. Is it practical? Absolutely not.

playing Survivalist

Stop running away from Medic Grenades
Stop standing while King Fleshpound shoots his chest beam
Stop taking dead people's weapons and selling them
Stop killing trash zeds in front of a Commando during Zed Time
Stop micspamming
Stop healing yourself next to nearly-dead teammates
Stop buying the Pulverizer
Stop buying armor on low waves
Stop buying the L85 Bullpup Rifle
Stop buying the Stoner 63A LMG
Stop welding doors around areas for no reason
Stop squeezing off shots with your HZ-12 to kill that siren across the map
Stop standing so close to the Trader when you are buying weapons (Support and Demo only)
Stop using the Hemogoblin
Stop taking all right-side skills as Field Medic
Stop firing the M79 at Scrakes
Stop spam-detonating C4
Stop setting Fleshpounds, Quarter Pounds, and Scrakes on fire
Stop using the Husk Cannon
Stop buying the Spitfire/Dual Spitfires (as Sharpshooter and Gunslinger)
Stop using Battering Ram

And finally,
STOP PLAYING SURVIVALIST

Originally I made this guide as a way to vent my frustrations with inept players on higher difficulties doing completely inane things. As the comments and criticism rolled in, I decided that it would be more beneficial for myself and the readers if I added in-depth explanations, reasoning, and information instead of "Stop doing x" line after line.

The Survivalist guideline is a joke. The rest are not.

Suggestions are welcome.
I stopped updating this guide a few months ago. Many of these guidelines are probably out of date. Don't get upset over some mistake in my guide, I probably already know it's wrong, and I don't care.