During the encounter with Vexallus in Magister's Terrace he summons small orbs of Pure Energy that when killed place a debuff on the player increasing damage done by 50% and causing the player to take a significant amount of damage over time.
In heroic mode the stacks from these buffs can be overwhelming depending on the group makeup so here is a little trick to avoid the debuff altogether, you sacrifice the damage increase but you take no damage from the debuffs.
Stand in the RED circles near the candlesticks and when you kill a pure energy you will not receive the debuff from them.