Better gear.
I was stuck here also, then my gear improved.
I use 2 tanks:
1 does taunt and AOE: 60k HP, 4k DEF
1 counter attack and shields: 37k HP, 6k DEF
They just tank it all the way to final boss on 12. Leave it on auto. Never have HP drop below 95%. No leech, no life steal gear.
Probably better ways of doing it, but this is no brain required.