Rival battle at Crystal Peak is ruining the game for me.

Playing on expert difficulty. Seriously. This is the first thing in this game that has made me want to quit playing. And I did for a week or two and I’m back and still can’t beat it. I have like 20 mons ev trained and iv maxed and I can’t beat him. My cap is lvl 61 and he has a lvl 65 mega tyranitar that hydro pump only does like 1/4 of his health to. Mamoswine is fast as fuck apparently? I’ve tried every combo that I have. Set up double toxic spikes, set up stealth rock, tried trick rooms. The AI knows exactly what I’m going to do. And when I learn what the ai is going to do to what I am going to do I go back and try to counter but nothing works. This shit is insane. What can I do or what did you do that helped here. Please. I just need any help. Nothing I have works.