20th level rogue with a ring of Greater Blink and a feat that improves sneak attack dice to d8s. Add flanking to help with True Seeing situations, and most sneak-attackable foes take 80-150 per round from him.
It's par for the group - my cleric does over 100 per round in most combats due to the Divine x/Righteous Might spells, the druid has some scary wildshape forms, and the wizard...well, he's a 20th level wizard with numerous blasting spells and metamagic feats. 'nuff said.
Edit: Oops, my bad. He's a 20th level character, but he's not a pure rogue. There are a couple of fighter levels in there and maybe Thief-Acrobat - I'd have to ask the player to know for sure. I think he said he has sneak attack of 8d8 at the current level.