I'v been in there once where we wiped once and killed it second try. The reason we wiped once was that I dident pull him far enough out of the pools, so he healed up. Reading your description it sounds like you might have done the same on your try. If he heals up, you will see it as "low DPS" as all the damage is basicly just healet.

Also make sure to start by pulling him up into theupper right corner(seen from the entry view) and then pull him down along the right wall. That way there is room enough to pull him a little extra out of the pools, to make sure he is not standing in it. You can also see it by watching the buff on the boss, if he is standing in the pool he gets a healing regen buff.