Level 2
Shoot First: This gets more useful once you have Shock Bow/Grenade Launcher.
Level 3
Aimed Shot: Obviously.
Level 4
Stealth Field Generator: Adds 30% to the cap, for 90%. Disarming Shot doesn't work on a lot of enemies.
Level 5
Grenade: This doesn't really matter too much. Energy-intensive but helps against groups.
Level 6
Quickdraw: Can swap weapons for different enemies, clear Disarm.
Level 7
Smuggled Stimulant: Before Called Shot, there are no abilities that cost more than 25 Energy.
Level 8
Second Shot: Again, obvious.
Level 9
Alpha Strike: Basically get 4 attacks in the first turn. (Shoot First is an auto-Crit now, +2 normal shots.) More, if the enemy doesn't have enough Shields to prevent Stun or Blind.
Desperation: Not really a fan of conditionals like this. You'll probably have enough Evasion to survive at 50% for a while, but you can't rely on being at 50% and you auto-heal at 50% once you hit Level 11. Better on the Defense build, I guess.
Level 10
Mag Binders: Reflex-based Stun is very useful. Alpha Stike can force it to Crit.
Can Opener: Only applies to Melee criticals, and most enemies don't have notable Defense.
Level 11
This one is discretionary. It really comes down to consistency vs spikiness.
Booster Pack: Usually actually best with alternate means of Flight, like
Wings or the Light Jetpack, because they don't cost Energy and last all fight. Probably technically better for Defense (because Hirudo can't auto-Blind).
Twist the Knife: Grenade Launcher (Flash) + Alpha Strike + Shoot First can often guarantee you'll Blind, then all subsequent attacks will be Aimed Shots.
Level 12
Specialized Combatant: Your damage is heavily reliant on Aimed Shot.
Level 13
Called Shot: Interestingly, Called Shot can be an Aimed Shot. Can Blind through Shields, in case Grenade Launcher wasn't enough.
Assassinate: Useless to you, unless you're also maxing Physique.
You'll probably want to pick up Concussive Shot by training with
Tanis. Concussive Shot
can Stun through Shields.