Are you doing it right? Click on the skull in the inventory so that the cursor changes into the skull, then right-click on one of the character portraits. You should be able to fire at least one charge. Since the Larkhon is close to the exit and behind the door, you can use the hit-and-run tactics. Fire one charge (or three, depending on your characters' mana level), run to one of the two forest maps (or just close the door), rest to replenish mana, then go in and repeat. It shouldn't take more than a few skull charges to kill the beast. Once it's dead, you can recover your items. Or, to be on the safe side, just unequip all weapons, since they're not much good against this opponent anyway.
Do tell us if you suceed!
Post edited February 10, 2013 by Charon121