believe me Tristan isn't unbeatable. All gym leaders on desolation, rejuv and reborn are meant to provide a challenge. Make u think of a strategy, instead of sending a pokemon out and relying on the type advantage. You just need to research on Tristan's team and make a team that counters it. Speaking as a person who fought Tristan before he was nerfed (with the snorlax and without the meloetta) He was much more difficult before, but I still managed to beat him with a proper team to counter his. Along with many retries.
If you need some help on beating Tristan, feel to ask around here. People would be more than happy to lend a hand.