How to fix Tristana
I'll be honest with you, I don't think that there is anything wrong with Tristana's kit and I think it is well designed.
That being said, I think that her damage output is just stupid, honestly Riot. I have nothing wrong with having a safer option as an ADC to play, but it has to come in the cost of not one-shotting every squishy champ.
Here are my suggested nerfs:
2 less base AD 0.05 less AD per level reduce attack per level by 0.15
Q - bonus attack speed from 30/50/70/90/110% to 30/45/60/75/90% W- no changes needed E - base damage from 60/70/80/90/100 to 60/68/76/84/92 R - no changes
This way, Tristana keeps her identity as a safer ADC option and still has decent damage, but has a less threatening mid to late game.