Without knowing your character build, I don't know if this would apply to you, but one trick I found useful with some of the enemies who heal when their health gets low is to use weak attacks/spells on short timers to whittle down the health slowly until just before they would start to heal themselves, then nuke them with the hardest hitting, long cooldown timer spells/attacks I had to try to quickly zero them out before they could heal.
That is, instead of say, opening with a fireball (in my case, I was playing a mage) which might take them down low enough that they start healing without getting killed, and basically 'wasting' the fireball, I'd just hit on them with magic missle until they were at like 50% or 40% of their health them slam them with the fireball-firewall one-two-punch, which usually would knock them dead in like 2 seconds, much faster than they could heal.
Doesn't always work, but works pretty frequently. Doesn't work quite as well on 'bosses' who might have just huge amount of hitpoints or large resistence.
Someone said that the warrior creature didn't work as well for them as ranger or mage. I was playing a mage, and I found the warrior creature to be useful sometimes, because my critter had the 'fatality' skill. There were times when I couldn't kill an enemy because of healing, but I could get them low enough that my critter would finish them with fatality. I'm not sure that applies with the pre-battle tower creature though, but at higher levels (like the Level 10 head), the fatality skill becomes quite handy.
