And lest we overlook, Karsa's got a built in resistance to magic that gives him an edge. Presumably, Mappo hasn't been just bopping Iccy on the head with his fist all this time, but actually walloping him with his mightymace . Icarium seems to need to get a good start to REALLY lay waste. There's a reference in HoC to a former companion, a Thelomen, knocking him out even as he trashed the Azath House where the Jaghut tree now stands.
So he's skillful and powerful, and eventually devastating, but not unstoppable at all times.
Hence, Karsa nailed him fast enough.
Which isn't to say that if Karsa had stabbed him and not killed him outright, he might not have gone ape****e and killed Karsa, Mappo, and most of 7C.
- Abyss, when in doubt, votes that Wolverine would kick both their asses.