I just beat Skeith of .hack//Infection. Tough boss, involved several dozen revivals of fallen team members and
lots of health restoratives (I blew through nearly 50 in the battle). When he can take 50 - 80% of your health away in a single blow and has a spell that targets the entire team with 100% accuracy, that happens.
The interesting, and awesome, thing about Skeith is that you cannot overpower him via level-grinding. Even when you've hit the level-cap (32, give or take) he's still a hell of a lot stronger than you, and you'll still need to be constantly healing to conquer him.
The fact that he has infinite HP is just plain annoying though...

Makes it difficult to gauge how much progress you're making.