Question about third trial of chaos. Need help.

I'm in act 3 doing the trial of chaos and at first it seemed hard but I thought I would be fine. The first two sections I figured out without too much trouble. It's the third one with the three cores that is driving me crazy. Apparently one of the monsters in this section prevents me from regenerating mana which makes it impossible to kill this mob. I end up running out of mana potions and then it's over. What in the world am I supposed to do?