To the best of my knowledge, Larrel is unkillable. I've never tried it, because he is essential to the plot (although maybe not if you go to Dorn's Deep first, allowing the plot to advance). Besides, he is an undead mage lord, do you think your average lvl 10 party can take him down?
Undead are generally immune to poison, cold, paralysis, enchantment, and instant-kill spells (other than Disintegration or Flesh to Stone, which are properly classified as transmutation spells causing permanent physical inability to be a threat). They are often also resistant to electricity.
What have you noticed in the feedback window? Does he have resistances to certain attack forms? Magic resistance? How low have you gotten him? Hurt? Almost Dead? If I am right that he cannot be killed, he'd probably stay at "almost dead" forever, no matter how much damage you caused him.
If you are serious about this, try:
A) to disrupt his spellcasting. Melf's Acid Arrow, Magic Missiles, Spike Growth/Stones, etc
B) to stun him so you can smash him for a while. Chromatic Orb might be your only chance at this.
I'm pretty sure that you won't have Disintegration or Flesh to Stone yet, and those spells might be the only ways to get him. Note that if you do not give him the Astrolabe pieces first and have him use the Heartstone Gem, the game will end.