i'd suggest a vamp gaze for dmg reduced and leech, because meph does swing a lot in close combat. i can't think of any armour with absorb, so there, probably just go with a shaft. that's what my lvl ** holy freeze merc uses, and with a potion or two, he usually survives just fine. Also a good weapon is unique thresher, i think it's called the reapers toll.
Go for vamp gaze, shaft/levi and ethereal ShaelShael bonehew (or reapers toll). Assuming your merc is always in close quaters with mephisto he should take physical damage. There is no one piece of equipment that gives all absorbs - i'm not even sure you can have all 3 absorbs in the helm, armour, weapon combo.
He's already using Vampires Gaze...and so far its been a huge let down...the biggest problem is heavy fire groups, like tons of bloodlords which takes time for me to kill and he dies very quickly to their fire.