Some more Will save, non-mind affecting/compulsion spells
Dictum (PHB) 7th level divine
Scrying (PHB) 4th level arcane/divine
Telekinesis (PHB) 5th arcane
Antimagic Aura (MoF) 7th level aracne
Backlash (MoF) 5th level arcane
Curse of Ill Fortune (MoF) 2nd level divine
Disquietude (MoF) 1st level bardic
Great Thunderclap (MoF) 7th level divine
Horizikaul's Boom (MoF) 1st level arcane
Know Protections (MoF) 1st level arcane
Shadowwell (MoF) 4th level arcane (shadow)
Shatterfloor (MoF) 3rd level arcane
Translocation Trick (MoF) 6th level arcane
Cast in Stone (MoF) 9th level druidic
Languor (MotW) 4th level druidic
Negative Energy Burst (T&B) 2nd level aracne
Negative Energy Ray (T&B) 1st level arcane
I would be worried with a weak Will save, even if I was immune to mind affecting effects. As an undead of course, the BBEG has a bunch of immunities (such as against stun ), deafening etc. does not really faze him, and if he is not a magic user (or heavily dependent on magic items) Antimagic Aura does not really hinder him all that much.
Telekinesis might be a good call, if you can throw the BBEG into some inconvenient place (such as into a lava pit etc. ), though.