[PF] Witchfire with class levels?

[OverdrivePrime]

TL;DR - Can I stack class levels on a Witchfire (http://www.d20pfsrd.com/bestiary/monster-listings/undead/witchfire) without bending the rules?

Hey all,
My players recently defeated, humiliated and killed an Annis Hag Cleric 6. It was meant to be a pretty challenging encounter, as the hag was backed up by 6 hill giants (party level 10), but they managed to pretty much dominate the hag and her goons thanks to a mix of good tactics and great rolls (and my usual terrible rolls).

Now they heard rumors in town that the hag made her lair in a cave not too far away, and they want to go find all the plunder. (The hag and her goons would regularly ambush ships, destroy them and take the goods back to her cave.)

So... I recently stumbled across the witchfire (http://www.d20pfsrd.com/bestiary/monster-listings/undead/witchfire) in the Pathfinder Bestiary 2. I'm definitely going to have the annis hag they killed come back as a Witchfire, but I don't see any notes about them gaining class levels, or even coming back with the creature's previous knowledge. Would the Witchfire remember its previous existence, or is it just a P.O.'ed fiery ghost hag?

At worst, it winds up a CR 14 if I bump the size up to large, add cleric levels and surround it with a half dozen or so Will-O-Whisps. I'm hoping to force a retreat and maybe create a recurring villain.

[CTrees]

There's no issue adding class levels. There's a section on afvancing monsters via class levels here: http://www.d20pfsrd.com/bestiary/rules-for-monsters/monster-advancement

Also, a CR 9 incorporeal creature that does 8d6 damage and a debuff on a touch attack? Ouch.

[OverdrivePrime]

There's no issue adding class levels. There's a section on afvancing monsters via class levels here: http://www.d20pfsrd.com/bestiary/rules-for-monsters/monster-advancement

Also, a CR 9 incorporeal creature that does 8d6 damage and a debuff on a touch attack? Ouch.

Thanks! That's just what I was looking for. Yeah, that thing is pretty brutal, particularly with backup.
But they're PCs! They can take it. :smallwink:

I've got a 10th level party with a Wizard, Ranger, Duskblade and Archivist (both converted from 3.5). We used to have a paladin/cleric on the team, but he and the druid moved to Korea to teach English, so the team currently has very little weapons against incorporeal undead. And advanced witchfire should really give them a run for their money, but they're smart enough players to retreat if they're getting curb-stomped.