Just get two globe of invulnerability scrolls. cast one at beginning, 2nd one halfway through. Profit.

with good preparation this combat it trivial. I preemptively positioned barrels of explosive and wine around the place where the fight would start. And I made a wall with crates at the side of the portal will spawn to prevent their ranged dealers because that what makes most people fail. lastly I used wall spell to force the melee enemy to take a big turn instead of directly coming at us. didn't loose a single HP.