Tristana
Super Rapid Fire
Lane harassment and consistent damage output.
Precision
Improved attacks and sustained damage
Fleet FootworkAttacking and moving builds Energy stacks. At 100 stacks, your next attack heals you and grants increased
MS .
Absorb LifeKilling a target heals you.
Legend: AlacrityTakedowns on enemies grant permanent Attack Speed.
Cut DownDeal more damage to high health enemy champions.
Sorcery
Empowered abilities and resource manipulation
Absolute FocusWhile above 70% health, gain extra
adaptive damage .
Gathering StormGain increasing amounts of AD or AP,
adaptive over the course of the game.
Adaptive Force
+9
Adaptive Force Adaptive Force
+9
Adaptive Force 
Health Scaling
+10-180 Health (based on level)
Why Fleet Footwork
As a heavy Crit stacking AD Carry, Fleet Footwork is a fantastic rune choice on Trist as it allows you to sufficiently sustain in lane and reposition with the burst of Movement Speed.
Why Absorb Life
Overheal pairs nicely with Fleet Footwork and Taste of Blood, allowing you to take short trades and come out ahead thanks to the extra bit of health going in. If you think you're going to have a hard time staying at full HP thanks to enemy poke, take Triumph instead.
Why Legend: Alacrity
Tristana is all about getting in as many auto attacks as quickly as she can. Early game this means a quicker [E] stack and late game is means more crits.
Why Cut Down
As an AD Carry Cut Down is the most reliable source of damage amp as most other champions in the game will have more health than you do.
WhyAbsolute Focus
Absolute Focus on Tristana will provide you with a ton of Attack Damage when you stay above 70% HP. This should be relatively easy to do with your **long attack range and Life Steal from Fleet Footwork, Bloodline, and items**.
WhyGathering Storm
Gathering Storm is your late game insurance policy, making you a 30min+ game monster.