Act2 Merc Auras:
Normal/Hell:
Offensive = Blessed Aim (raises AR, not that useful)
Defensive = Defiance (raises defense, somewhat useful)
Combat = Prayer (heals your health, pretty much useless)
Nightmare
Offensive = Might (increases attack damage, this one is the best)
Defensive = Holy Freeze (freezes all enemies around him, not bad, just explodes too many corpses that could be used to CE)
Combat = Thorns (not too good)
In short words, go to nightmare and hire the offensive merc.