Lancers
The 3rd boss (The guy Duke) takes a large amount of mana from you when you block. If you have a mystic, get him/her to lay mana skittles, or you'll find yourself not blocking to try and get mana back. It's not too painful, but watch out for a move that takes 80% of your HP (or was it 90%? Either way your screen will go Silent Hill)
Yea, it's has to be removed before it's timer counts down, or you lose 90% of your hp, not usually an issue unless it's a mystic with 3 melee dps who won't pick up red balls, or if your priest sucks.