Depends on the merc. An act 2 merc is face to face melee, and most of the minions, or say, Mephesto's charged bolt, would more than likely kill him before he could help. An act 3 merc stays away, but the elmental damage is low. For all purposes in your case of having a zealer, an act 1 merc is best. she stays away from most of the fighting, does better damage than an act 3 merc, and at the very least wil lcast inner sight before she dies, which will take down the defense of all targets in the area substantially.
We normaly make trist with a zealer, a smiter and a frenzy barb. Two act two mercs and a rouge. The chars and gear are optimized to provide as many auras as possible. The odds to cast lifetap for the chars is 5%, 5%+15% and 5%. A nice bo does the rest. Normaly the mercs survive.