YOUR PLAYERS WILL LOVE THIS
Instead of giving XP for fighting monsters,
GIVE THEM NEGATIVE XP FOR IT.
What I mean is, for every point of hp they lose, THEY ALSO LOSE 1 XP.
So, if the cleric loses 3, the thief 2, and the fighter 5 hp over the course of the fight, the whole party loses (3 + 2 + 5) = 10 / 3 [number of PCs in party] = 3.333 round UP [hahahah] = 4 XP each.
Don't worry about the hp getting healed. What matters is they lost it. If they get hit, get healed, get hit again, they're JUST LOSING MORE HP.
Get outta there. It's a bad scene.
I'm doing this for my 4E thieves guild game next time. We'll see how THAT goes.