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.