Sigh...I've learned to hate this guy. Guild run wiped on him for 3 hours last Naxx run. We'd downed Patches the previous night, so Grobb was the first and only one we ended up fighting the entire night. Tried kiting him on the outer wall, on the inner wall, no real difference noticed. There was a long debate on how far to kite him exactly, how big the rings get, what ranged should do, what melee should do, etc. We wiped on him so much, the richer members of the group ended up donating gold to the poorer members for repairs :P Think I lost track once we reached the double digits.
Anyway, the closest we got was someplace past the 25% mark, and it was just the speed which people were getting injected was killing else. Our designated callee was yelling a different name about once every second, and it killed our dps and was so chaotic, it just wiped the attempt.
We'll get him eventually. I hope.