I remember proccing Glacial Strike FOUR times against him and he was still not dead... I go water and ice and physical damage, and I went with that trait that increases the TEC and the Blue Gem effectiveness. I think it was called Protector trait at the start of the game.
I think matching healing gems and blue gems are key. Try not to get overwhelmed. Also, I would try and get all the yellow gems, those that charge the physical damage before him. I think I got him on the 4th or 5th try. I basically outlasted him. That's how I did it. Maybe you can let us know how it is going? Did you beat him?
I was wondering if Mohtir was a placeholder character for some other MC? I see we will have plenty of interaction with Aoba for example (is a love interest), but Mohtir is not. Perhaps a main character got cut.
If you're even able to cast Glacial Strike four times, it means you and Mohtir have both been extremely hyperdefensive. Mohtir is not actually hard to beat at all if you just aggro rush him before he can get off his physical resist gems.
TBH I am getting the impression the people struggling against Mohtir are all unaware that he can create physical resist gems. Do people just not read the gem enchants? You're gonna have a really tough time in any fight that way.