I was matched up with her and at first it seemed like I was going to wreck her because I went 1/0, then she had her ult and kept murdering me over and over, and got fed. How do I prevent this from happening?
keep your distance early. be shy. poke her to get her attention, then flee as fast as you can in the opposite direction. Do this until she's low enough for you to burst her faster than she can burst you. Remember: STAY OUT OF RANGE. As much as possible. And i don't give a rats ass what the rest of her team is doing, you build a metric s*** ton of mres because her ult deals around 1200 on its own (with an aptimum build) --- I'll stomp you in the face with my sandals enraged and tonight we shall rhyme in the shade~King Leonidas
You should not even be having this problem to begin with. Mordekaiser is an awful champion, and Tristana is not effective as an AP Mid.
Generically, when fighting bad players who play AP Tristana (because good players do not), avoid fighting her inside a minion wave, and CC her. And you obviously have CC, because you are playing an actual mid-lane champion and not something silly like Mordekaiser.
The cooldown on all of her abilities is too high for her to duel effectively, so if her first burst doesn't take you out, she will have to rely on AAs. The passive on Banshee's Vein grants increased health regen to combat poke. It also has a comfortable build path and good stats, MR being one that perfectly suits your match-up against Trist.
Mordekaiser's abilities have good base damage and low cooldowns; all this, together with his shield passive, and you will find yourself in a duel between a champion who's good at dueling and one who isn't.