This recourse is intended to be a more precise and condensed informational package then Lavikon's guide. (Which is phenomenal if you have not checked it out.) In addition, it contains brief "blurbs" pertaining to my personal experience playing the game.

The Basics

• You get up to 2 "tier 5 special abilities".
  
• You get a maximum of 1 "tier 8 special ability" from a category you have a tier 5 ability in.
  
• Each category has 10 ranks, and you always get the base abilities for that rank even if you are not "specialized" (have a special ability from) in it.
  
• The first category you receive a special ability from determines your first title.
  
• The category which you select your tier 8 special ability from determines your final title.
  
• There is no resetting of your chosen special abilities, so whatever you choose the first time, you are playing with for the rest of the game. Note: You can get mods that allow you to retrain your pilots.

Brief Note: Personally, I'd suggest simply ignoring these, as for me they were a bit misleading.

rank 5: Multi-Target
Description: Active Ability. Fire upon up to 3 targets in the same action.
Comments: Cannot use Precise Shot morale ability with this. Cannot target specific sections of disabled mechs. Personally, I dislike this skill, but is good in instances of multiple-knockdowns as well as when a mech has weapons with multiple maximum ranges, especially for those with minimum ranges.

rank 8: Breaching Shot
Description: Passive Ability. Ignore Cover & Guarded IF you fire a single weapon at a particular target.
Comments: Synergises well with Multi-Target as long as you fire only a single weapon at each target. I actively avoid getting this skill, as you will always do more damage firing two or more weapons at the same target. I find it works best with multiple high-capacity LRM launchers (15 or 20), but is good for PPCs to a certain degree, and AC20s if you are close enough and the target Braced or is within damage-reducing terrain. (Would also be good with the Gauss Rifle.)

Special Note: Alright, battle scaling dependant upon the mechs you field is hotly debated in the community and I'm not going to open that can of worms, so here it is: I have personally found these two skills much more useful in non-Primary missions when I fielded a variety of weight classes instead of rolling in with 4 assault mechs. Either this is true or it is not, go play and find out and post in the comments so others can learn from your experience as well. Thank you.

rank 5: Evasive Movement
Description: Passive Skill. +1 "Evasive Pip" for any movement of a certain distance or more, up to your maximum number of Pips. (4 initially, more with higher piloting.)
Comments: Improves survivability for any mech that ever moves, though ironically more useful for slower mechs and much less so for quicker ones that reach their maximum number of Evasive Pips more easily.

rank 8: Ace Pilot
Description: Passive Ability. Can perform "attacks" either before or after moving.
Comments: Works with Multi-Target and Sensor Lock, does Not function with melee. This ability adds both a great deal of defense as well as offense, most likely more then any other ability.

rank 5: Bulwark
Description: Passive Ability. Gain Guarded on turns you do not move.
Comments: Great survivability for any mech that does not move every turn. As long as you remain in the same spot this ability activates, even if you change facing or even make melee attacks as long as the mech is in the same location after the attack as it was before.

rank 8: Juggernaut
Description: Passive Ability. -1 hostile Initiative from melee hits.
Comments: Limited usefulness, at this point I avoid it. There are only 5 inititiative phases, and any target that is already acting in the last, for any reason including knockdown, is completely unaffected. Much more useful against light and medium mechs to knock them into an initiative phase that they can be fired upon by heavier mechs before they can act.

rank 5: Sensor Lock
Description: Active Skill. -2 target Evasive Pips, and guaranteed visibility until end of round.
Comments: Can target "red" non-visible targets. Adds a great deal of versatility to almost any mech other then those using Indirect Fire. Less useful with high Gunnery ranks and better equipment, but always remains pertinent.

rank 8: Master Tactician
Description: Passive Ability. +1 Initiative, -1 Stability Damage every time you Reserve.
Comments: Good for all mechs, especially Assaults. Adds an insane ammount of versatility for the same reason Juggernaut is so restricting, effectively allowing any mech to act 20% sooner in combat, in addition adding a substantial buff to reserving. (I believe it is also 20% of unstableness that is removed each time you reserve, as well as the Unsteady status if you have it.)

Note: The numbers on this chart do not stack, they are the totals for that given rank.

• Ranged/Melee/Indirect: Bonuses to hit chance for different attack types. +3 Indirect means no penalties when firing in an arc.
  
• Called Shots: The chance to strike a parrticular portion of an enemy. For example, the normal chance for a head shot is 1%; this improves to 2% on a Called Shot, or 5% if Improved (+1), up to a maximum of 35% with Mastery (+2).
  
• Evasion: The maximum number of Evade Pips a mech can obtain.
  
• vs. Unsteady: (Unsteady Threshold) I figured it out finally! The unsteady bar is broken up into segments, this increases the number of segments required to apply the Unstable property (which has a variety of penalties including disabling sprint).
  
• Sprint: An increase in the maximum distance travable when taking a sprint action.
  
• DFA_SD: Death From Above, performing a melee attack using Jump Jets. This particular skill reduces the Self Damage suffered to your mech's legs when you perform a DFA.
  
• Survival: A bonus to a pilot's chance to survive being Incapacitated in battle. You will find out if a pilot perishes or survives once you return to your ship.
  
• HP: Hit Points/Health/Hits/whatever - The number of injuries a pilot can suffer before becoming incapacitated.
  
• Recoil: Uncertain what this means, my best guess is reduction of the penalty from firing the same AutoCannon in consecutive rounds.
  
• Overheat: (Overheat Threshold) The maximum heat before a mech shuts down does not change, only when the mech starts taking internal damage from heat. (And there seems to be a glitch, that either it does not function completely as advertised, or a pilot will still say they are taking heat damage when they are not.)
  
• Minimum Range: For weapons with a minimum range (LRMs, most ballistics, PPCs), this allows you to fire the weapon closer to your mech than normal, and also reduces the penalties for firing at targets within a certain range threshold.

A list of the different mech roles, and the builds I believe work best with each. These suggestions are only in regards to the Special Abilities, as all four categories can literally be ranked up to 10. (My last main character, before I restarted...)

LuRM Boat: <nothing but LRMs> Lancer (multi-target), Striker (glass cannon), Vanguard (versatile).

Meleer: <where is my mech-hatchet, and why doesn't the Black Knight have a sword?> Brawler (dedicated), Gladiator (multi-role), Sentinel (spotter).

Mobile: <speed is life, and backstabbing so much fun> Flanker (hit & run), Outrider (nesting), Skirmisher (multi-target).

Nester: <for soaking large ammounts of damage or offensive mechs that prefer to remain stationary> Lancer (multi-target), Outrider (mobile), Vanguard (versatile).

Offensive: <exceling at dealing damage> Flanker (hit & run), Gladiator (ultimate), Lancer (nesting), Sharpshooter (multi-role), Skirmisher (mobile), Striker (versatile).

Sniper: <dealing heavy damage at extreme ranges> Flanker (hit & run), Outrider (versatile), Sharpshooter (multi-target), Vanguard (overwatch).

Spoter: <making targets easier to hit for their lance mates> Recon (hit & run), Scout (versatile), Sentinel (melee).

Tank: <surviving being focus-fired upon by hostiles> Lancer (multi-target), Skirmisher (evasion), Outrider (ultimate), Vanguard (versatile).

Gunner: Gunnery 5. *sigh* I am sorry, I really do not care for these, but they do make great harassers with more mobile mechs that carry a variety of weaponry with different ranges. I almost always run mine with both direct and indirect-fire options. Be careful of heat buildup however.

Pilot: Piloting 5. As a word of warning, Jump Jets produce heat dependant upon multiple factors including the distance jumped. As many things, it is unclear at best the mechanics of this, but the effect is very pronounced: Firing a weapon that normally displays no heat gain in the bar thingie after jumping will fill up said bar really quickly unless in water, cold environments, or you have an absurd number of heat sinks.

Defender: Guts 5. Best for mechs with heavy weaponry that you can sit somewhere and go to town. If you are making a melee attack, be certain the target is adjacent to you (and you might have to turn to face them too, uncertain about this).

Tactician: Tactics 5. Honestly, this is my first get for any spotter mech, and can also be great for slower mechs that take time to get a line of fire. All but useless on mechs that have indirect-fire options.

Skirmisher: Piloting 5 & Gunnery 8. Err... Yea. I have to highly recomend against this combination as the skills are exceedingly counter-intuitive. You would want a mid level of mobility combined with three extremely heavy weapons that do 50+ damage, with a maximum of 6 Evasive Pips that have reduced effectiveness each time you are fired upon. I like to believe I am pretty creative with unorthodox tactics, and I am uncertain if I would be able to come up with a mech build for this that is on par with other skill builds.

Lancer: Guts 5 & Gunnery 8. I prefer other builds over this one, but it certainly has its uses. What I use mine exclusively for is a Heavy LRM boat with three LRM 15's mounted, and even then targeting a single mech with two of them will always do more damage to the target. It can be used to knock down multiple mechs, but what I find it more useful for is softening up mechs so other mechs have an easier time coming in for the kill, both stripping some armour as well as removing an Evasive Pip to make them easier to hit.

Sharpshooter: Tactics 5 & Gunnery 8. Not recomended for snipers honestly, which are most effective firing two or more weapons on the same target. (One to punch a hole in armour, the second/others to deal internal damage.) I do not use this build anymore, but it would actually make a good multi-role mech. You would want to be mobile to be able to get out of line-of-fire and hop from terrain that offers cover, and I'd actually suggest lighter mid-damage weapons for this, those that normally deal decent damage but mean very little against Guardred targets.

Flanker: Gunnery 5 & Piloting 8. Inane title, but actually a lot of fun. Best with high mobility mechs that manage their heat well.

Outrider: Guts 5 & Piloting 8. The ultimate defensive mech, and crazy fun to use. Best with mid to high mobility mechs that are capable of dishing out decent damage. Also phenomenal with heat management, just sit in the same spot for a turn to gain Guarded to help vent heat, then can fire upon the same target before moving/jumping or just staying put the next turn.

Recon: Tactics 5 & Piloting 8. I have fallen in love with this one as my spotter, best with a medium or good mobility heavy that can mount some decent weapons, just be careful of heat build up as you will want to move every turn, either before or after firing/spotting.

Gladiator: Gunnery 5 & Guts 8. The ultimate attacker, works best with Heavies and quicker Assaults. Besides the decent mobility, you will want a nice variety of weapons and ranges, my recommendation being LRMs as well as Support weapons and a complement of closer range medium lasers or SRMs.

Brawler: Piloting 5 & Guts 8. I am sadly forced to recommend against this build, as it's only real use would be a dedicated meleer and Light/Medium huntress that simply are not present in high-skull contracts. If you desire to give it a go, I would say get a balance of high mobility, Support weapons, and mech mods that enhance melee, especially the ones that reduce DFA self damage. You would want to gear towards DFA as it does the most melee damage, and it has the farthest range.

Sentinel: Tactics 5 & Guts 8. This melee build works phenomenally for Light mechs and Cicadas, however it does become pretty useless with higher skull contracts, especially if most of your pilots have high Gunnery. You don't have to worry about heat, the only weapons you mount are Support which can be used outside of melee at extremely close ranges, and you can mount more armour due to the lighter weapons. So you are aware, Flamers only can be used 6 times in a single combat. (I had not noticed that until the first time I ran out.) Take caution however, you really do not want the target or other mechs attacking before you move again, and be certain to check the number of Evasive Pips you would receive from moving to the spot you make the melee attack from. Finally, only DFA if you have equipment in your legs you can replace.

Striker: Gunnery 5 & Tactics 8. Highly versatile yes, however I tend to stay away from this build, the best use of which would be piloting mechs that do not get attacked often if ever, or have enough armour that they can take the hits they are subjected to. In the end, I have found this build most effective with an Assault LRM Boat.

Scout: Piloting 5 & Tactics 8. The speed demon(ess?), they excel at going really fast and very little else, which is mitigated by a maxiumum of 6 Evasive Pips at Piloting 10. You will want to move every turn with this build, at the end of which you can fire/melee/spot/brace, or you can simply sprint a very long distance. Would actually be good while piloting a high-mobility medium mech and less useful for lights, while disastrous for slower mechs that have trouble escaping direct fire.

Vanguard: Guts 5 & Tactics 8. This is probably my favorite build, with a wide variety of versatility as well as good defensive options. I consider it the dilettante build, not really excelling at many things, but being able to handle a large variety of situations. It can be used for any role, but is not as good as other builds for most of them. What it does better then others is your beloved Assault mech pilots, allowing them to act in the heavies phase or reserve and remove instability, while finding a nice spot to sit in and erase targets with heavy weapons fire and taking reduced damage as you do so. With their massive amounts of armour and capability of eradicating more mobile mechs that are capable of getting within their rear arc, mostly you just have to worry about being in a cross-fire with spotters in front of you and long-range heavy weapons firing from behind. Sustained heavy-weapon fire can also chew through your armour, very, very slowly.

Blah blah blah, you know, the usual "thank you everyone" and stuffu. Seriously however, I just did a massive revision now that I have much more experience in-game, and hope the information here is much more balanced at this point. I also rephrased certain things to be less antagonizing, for which I apologize. Finally, if anyone has comments or suggestions regarding this guide, I greatly appreciate those whom spare the time to do so. We, the players, are what make games great, for ourselves and others, and for me, I always enjoy being able to contribute to the community.

2019.12.17 Update: Hey, I finally did an editorial update, just fixed grammar and and spelling errors is all.