If you want to keep your Merc alive, life leech, res, and damage reduction is what you need. Having a higher level Merc is good too, the same build with a low 70s vs a mid high 80s merc was a lot different farming the same areas.
Armor should be Treachery, but a good Leviathan isn't bad either. Helm should be Tals Helmet till you can upgrade to Gaze or Andys