My guild downed Illhoof twice before they "fixed" him. I have to admit he was a joke before. However, we've been having trouble lately on him. Before we used the "AoE imp clear, attack the chains, burst DPS him down" method. Now most of our kills on him are "One person alive, go go go go, yes... whoa that was close" kills.
Just wanted to know what other people found effective now?
Kill his Imp instead of OTing it. React to Sacrifice faster. Fire Elementals and Seed of Corruption spam. It's just longer, but that's it. More of a gear check. A warlock helps a lot, but a paladin/mage would be fine too. If you somehow have little/no AoE at all, you're going to be in trouble.
Edit: And yeah, we kind of have a big Karazhan catch-all thread for this stuff, since a lot of the same questions come up repeatedly, and the zone isn't that big.