Sup. I'm Assailantz- here Summoner name natraniel. Not that high of an elo cuz too many trolls in the league. w/e. This is my guide. Works most of the time for me mid. I haven't lost mid with AP Trist in ranked once yet. She's a dirty burst champ that everyone hates lol. IMO worse than when everyone bashed on AP Yi. She has insane damage but you have to use her right.

You want to have magic pen, CDR, AP, mana regen. Nuff said.

First of all what you want to do is utilize her E whenever the enemy comes in range at all. Just spamming your E shouldn't kill your mana too quickly. It does so much damage it should take your enemy down 1/6th of their health minimum against an AP mid (unless they have magic resist ofc).
Make sure you max out your E first.
The correct sequence to burst down your enemy champ in ONE burst usually if you have enough AP is, E -> W on top of them -> ignite -> AA once -> R -> then W to escape.
If the enemy is far in a distance or under turret and you can go into range by jumping with your W you want to, W -> E -> ignite -> AA once -> R -> W . Make sure when you're jumping towards them they don't have stuns under turret or in general. You don't want to jump in at the wrong moment, have someone stun you then give them enough time to have other teammates assist them.
When trying to escape you should use W, and in rare cases use R, when the enemy is close enough to engage. Her passive helps because by level 13 Tristana should outrange every champion so don't be afraid to use your ult to get away. The CD on her ultimate is also not too bad. Should be around 36 seconds late game with ionian boots.
Take Q out of the equation until later in the game. It will hurt turrets a lot later as well always use it to try and take down a turret.

This is just a basic build that should be very easy for mostly anyone. I don't have too much to add just make sure when you play her be aggressive in lane with her E but make sure you have map awareness and ward your lane.
And just like I wanted, this guide is short and simple.