Cold/light ss maxblock (for more safety) sorc is able to do ubers, not as easy as a smiter or trap/kicker, or other melees, but fine enough. Minor Ubers are easy, Lilith can kill ur merc sometimes, but just rejuv/antidote him in the right moment. With static and charged bolts/lighting or whatever u decide to use as hybrid, it's fine. In Ut, Dia and Baal are easy with lighting + infy merc, just separate Mephi first and let him lost in some corner with teleport, then go for the others; and then separate dia or Bhaal, and don't make a chaos with minions. When Ubers are dead, merc can kill the inmune minions left. For Mephi you just 'need' marrowalk boots, so u can cast boneprison to make mefisto stand, life tap him, and give the merc a gear he can tank ele dmg with, thing that is not that complicated: fade him, give him a coh/kira, and make his infy in a fast base like GT or Tresher. Should apply his Infy CB enough to damage Mephi pretty ok, and leech enough from lt, and if not, rejuv him. Keep ur sorc away from danger or if u want more safety, fade her and give her a sanctuary vs Mephi, while you cast Blizz/Frozen orbs or the cold skill u have chosen as hybrid on him.
Key for merc to survive better in UT is to cham his helm (not needed if using a kira vs mephi cos it have CBF itself, but needed when using vgaze or other).