Lamatar Bayden is a frost wight, which is a CR 4 creature before you add his Ranger levels. Barl Breakbones would need to at least be a Level 15 Wizard to use Create Undead to create a frost wight (equates to a mummy in regard to required caster level). Animate Dead only creates skeletons and zombies.
I guess you can hand-wave the rules discrepancy, but have any of you raised Barl to a Level 15 Necromancer and not TPK'd the party?
Raising Barl to Level 15 makes him even more powerful than Mokmurian, as written. As such, you would raise Mokmurian's level up a couple of notches. Then again, that puts the party in serious TPK danger (a second time).
What to do?
Isn't their some ritual option for magic which would allow for greater result in return for more time and material spent?
I've always seen the spell described in the books as the handy battle spell, sure proof and battle tested but if you let some time and peace to a magician he should be able to improve on that.