yeah a good act 2 merc is what i usually went for with lots of Life leach. Also if you want to help him out you can get static feild which helps him alot.
Think not disdainfully of death, but look on it with favor; for even death is one of the things that Nature wills.
Make a Light sorc with eth infinity on your merc and you can dmg everything...altho it wont do much...he would do most of the dmg to lite immunes since his weapon itself will do 1k dmg.