In that case I can't say anything specific because I don't know those rules, but I still think that planning for a villain to escape alive is risky unless the fight is totally rigged.
Although if I were running this, I might just have him attack again even if the players killed him. Ideally, he'd run away the first time, they would make sure to kill him the second time, and then they would be quite surprised the third time. The original body would still be there, and so the players might end up with three or four dead Alans before they're done with the dungeon*. Where the extras came from is not explained. This also creates an opportunity to bring Alan back much later in the campaign, because the players might have forgotten about him but he hasn't forgotten about them.
*If they bring back evidence of more than one dead Alan to the quest-giver, he will not want to pay them because they can't prove that the real Alan is dead.