I guess level/AA varies, but for me, at 110, even the tank merc has no issues keeping agro off of me. The only thing I have to watch for is snare, Blessing of Ro, and Storm Strike/Firestorm all being cast early in the fight. If I don't space them out a bit, I'll definitely grab agro for a bit, but after that, no amount of DoTs, heals, or nukes gets agro from the tank merc.