General overview

Aak is most famous for his buffs (that require his target to survive being shot 15 times... but that's not important right now). Also, he will die if he's not constantly attacking and doesn't have an external healing source. Sometimes his targeting priority requires ridiculous positioning to get him to shoot the person you actually need him to shoot. But once you figure him out, his buffs are very good. Buffs aren't really necessary in standard game content (and a lot of other units can buff without inflicting 7500 friendly fire damage), so he shouldn't be high on the priority list for new players, but it's worth it to come back to him once you're looking for some fun projects. It's also worth noting his DPS and crowd control are nothing to sneeze at, either.

E2? He requires maximum investment to do his job well, so keep that in mind before building him. Early game players who want to play around with him are better served finding him in supports until you have more core units built.

Talents

Talent 1 (Drug Cocktail Administration) gives each of his attacks a chance to trigger one of 4 effects: Heal himself, crit, slow the target, or stun the target. On average, about half of his hits will be crowd control, which is very useful. (The stun only lasts for a second at E2, so don't expect it to be much more than an enemy disruptor.) The self-heal is also useful, especially when taken with talent 2 (Pharmeceutical Diffusion), which increases any healing effects that he receives.

Skills

S1 (Rapid Fire) gives Aak a massive ASPD boost that allows him to proc his talent at blinding speeds. It's often overlooked but is a genuinely really good skill that allows him to hold his own as a unit and not just a buffer.

S2 and S3 are where Aak's buffs come into play. Both skills require him to shoot the target 15 times with 500 ATK (for 7500 damage total), after which he will buff both the target and himself. S2 (Type γ Stimpack) gives a HP and DEF buff, while S3 (Durian-Flavored Stimpack) gives an ATK and ASPD buff.

Masteries

All 3 skills get a lot out of masteries. Most players will find S1 and S2 overkill, though. (If you're in a position to be doing Aak S2M3, I don't think you should be looking here for advice!) S3 definitely needs M3 to function at its full potential and be competitive with other buff skills.

Modules

Module X increases Aak's SP recovery rate by 0.25/second while he's above 80% HP. Most people use Aak as a helidrop before retreating him, so this will help him get his skill online faster after deployment. The module upgrades target talent 1, so they're not really necessary if you're only using Aak for his buffs. Heavy S1 users will appreciate these upgrades, especially since they add in a chance for him to trigger all 4 of his effects at once.