1) The devils, in fact, already have the knowledge from the book. This operation is strictly to keep the knowledge out of our hands. So backing out of the fight is still an option.
5) Storming the Nine Hells to retrieve the book might be easier than defeating him.
I'm missing something here. Why would retrieving the book from the devils change anything? This guy was hired by the devils to keep the book's information out of mortal hands (or your hands, at least), right? It sounds like nabbing the physical book just makes them more determined to go after you.
6) Having two solars aid us wouldn't change the balance much
Perhaps he knows
dismissal, banishment, or the like?
Can you gate in templated creatures?
* Celestial ancient silver dragon (34 HD, 20d8 breath weapon, SR 29, casts spells as 15th lvl sorcerer)
* Fiendish corpse tearer linnorm [MM2] (28 HD, paralysis breath weapon, SR 39, casts spells as 17th lvl cleric, each claw attack bestows 1 neg level)
If I was designing an epic psion badass, he would have Psionic Meditation (natch), Quicken Power, Multispell/power x2, and would know both
schism and
hustle. That's four powers a round if he has the luxury of not moving - 1 normal, 1 from the
schism, 1 quickened, move action to regain focus, 1 more quickened,
hustle for an extra move action, which is used to regain focus for the next round. You could add in
fission as well for even more fun.
Actually I would give him at least 3 Multispell feats, so that he could also use an immeadiate power when it's not his turn (
energy adaptation, evade burst, etc.). If he still had epic feats to spare, add in one or more Epic Psionic Focuses and some other metapsionic feats so that he can further enhance the 2 quickened powers each round.
If such a character gets to act, your group is quite possibly dead. But, that's epic levels for you.
But, everything special that he can do requires his focus. Your group is probably already aware of this, but regaining focus provokes an AoO. If you can get your fighters or summoned beasties to stick to him, it might slow him down some.
And
shatter mind blank is worthless. As written, the target gets PR, a saving throw, the caster has to succeed at a dispel check, and its range is 30 feet.