You need any source of Crashing Blow, Open Wounds, Cannot be Frozen and Life Tap, and high Light Res (and Cold Res) vs Meph
Black, 4x Pdia Shield, Draculs, GFace/Giant Skull, and Gore Rider/Goblin Toe are the budget items.
VS Meph, Conviction, or Salvation if you hope your merc survives (every two point to Resist Lightning give you + 1% Light Res)
VS Diablo, anything
VS Baal, ghosts are annoying, either use Sanctuary or equip Azurewrath on a5 merc