My merc is poor so he uses shaft, eth gaze and double AMN bonehew. Duriel's shell also worked well for the resistances.
My ladder merc is extremely poor and constantly complains about his gear, but that's tough shit for now. When you are demon bait your choices in life are slim...
I have rarely used a A2 merc for more than a minor meat shield and a useful aura. I try not to depend on them to do the killing for me. My major concern most of the time is for him to stay alive. I don't think it is the over all best setup from a min/max standpoint. It is however the ideal setup for how I play:
Weapon: Eth Reaver
Helm: Eth Up'ed Gaze (UMed)
Armor: Eth Up'ed Shaft (UMed)
Depending on the character I am using I can switch out the weapon for a Doom Cryptic Axe if need be.