I have a 200FCR Nova Sorc with fairly good equipment/skillers (just missing infinity, which would obviously make a big difference), the main issue is the speed of killing bosses and the stuff that remains immune.
If you're hardcore taking a Holy Freeze Merc aids survivability a little, making it less risk to teleport into the middle of mobs, your merc will have less damage without Might aura though.
Once you're really high level the majority of XP will come from killing DIA/Baal.
You can actually get surprisingly good XP from players 8 Ancient Tunnel, and nova rocks there, as there's no big bosses.