Hi, I am a french trist player, in this guide you will find some tips I've learned after playing trist and getting 100k point, I am definetly not he best trit EUW so after reading this you won't a faker tier trist, but you will have in my opinion good bases.
If my sentences don't make any sens pls tell me, I'm still learning english. Also if there're any useful tips I forgot to mention about trist pls let me know :)

Your lvl 3 powerspike (or 2 if you play with hob)can allow you to win most of the 2v2 fights but it's not a reason to always go in. Now you should only jump in when your support have engaged the fight, DO NOT jump in if your support is behind the fight or is low because you will always need him.
Let's say that a fight have just started, remember to ALWAYS E the adc, except if the support is low and the adc is too far so you can then desengage. If your supp manage to get a stun on the adc and your bomb explode then it will most of the time be a gg. Also remember to heal if your support is getting low in a fight and it can saves him.

Trist also do have a huge lvl 6 powerspike, if you have your lvl 6 before the ennemy bot lane and you are not too low jump in, E and then R the adc, try to use your R behind him (even if it cost a flash) so he won't be able to run away, remember your ult is very strong and a full combo can pretty much kill every adc in the game.
You also can do that to ennemy support if he is alone in lane but be carreful, the ennemy adc/jgl might be close so ward.

Try to be lvl 2 before the ennemy mid laner, if he is not playing safe go in and kill him, with hob it will almost always work and if you add an ignite then that's a gg.
Lvl 6 a full combo can kill every mid laner under 3/4 hp BUT be carreful before those kind of all in, always ward and then kill your ennemy.

Tristana has a very strong all in potential BUT try not to jump in every fight they start, if you're stun it's over. So wait for your tank to tank every stun and then go in only if u see a target that you can easily kill since it will reset your E and allow you to replace yourself in a safer position. You also have a very long range so you can detonate your E even if you don't jump in.

KNOW YOUR LIMIT trist have a lot of dmg but is very squishy, an adc in mele fight is always bad, so know when to go in and when not to.