As far as the type of A2 merc I go with this:

Any Melee damage character (wwbarb, Multi/Strafe Zon, Zealot that doesn't use Dracs, etc.):
Might merc with Eth Reaver

Casters (Hammerdin, Trapsin, Sorcs, etc.):
Defiance Merc with Eth reaver

Others (Zealot with Dracs, Pretty much all Necros):
Might merc with Doom Cryptic Axe.

I am pretty much 100% PvM so keep that in mind. I have no clue what setup is ideal for PvP mercs.