So, we're currently a mid-level party averaging ECL 13-14 in the middle of a factory-like tower that seems thematically aligned with iron and fire-based creatures. It's been a very long dungeon so far, and we've made slow progress over the last few sessions. Our party is generally extremely optimized (admittedly almost entirely my fault), and the DM has previously dealt with that by sending vastly higher CR monsters at us, leading sometimes to far too difficult encounters and other times having encounters as complete cakewalks when they shouldn't be. It's generally been okay so far though.

Well last session, the DM has informed me (out of and in character) that we're going to be facing a pit fiend in this dungeon. Now, one of the spellcasters we've already faced in this dungeon has given us a taste of the power of Blasphemy, and I have to say, I'm not looking forward to it again, especially not as an at-will SLA. Out of the entire party, only me and maybe another person are immune to the spell (Evil alignment), and since we're so much lower level than the encounter level, Blasphemy is pretty much guaranteed to paralyze half the party and daze-lock/weaken most of the rest.

Party:

Me, a level 14 Fiend of Possession/Ardent/Thrallherd acting through my Half-Minotaur Dungeoncrasher Fighter/Warblade thrall (though party thinks I'm the minotaur). The thrall has the Power Attack/Shock Trooper/Knockback combo, while my actual character has some useful powers in Astral Construct, Expansion, Major Creation, Psionic Dominate, and Metamorphosis (with Metamorphic Transfer) all at ML 10. Unfortunately, the DM has ruled that since we're in some kind of demiplane rather than the Prime Material, the Ethereal Plane does not exist here and thus I'd prefer not having to unpossess my thrall since I likely won't be able to get back in for quite a while.

A level 12 Wizard/Incantatrix focused entirely on blasting. He has metamagic'd Orbs up the wazoo, but is extremely fragile (he uses almost no buffs at all). The player isn't even with us half the time, making relying on his character a risky proposition at best.

A level 14 DMM Cleric/Entropomancer who buffs heavily. The Entropomancer is houseruled to only lose one caster level, and the DM promised him a Sphere at 17th level if he takes the class . He can persist 3 spells a day (usually Righteous Wrath of the Faithful, Mass Lesser Vigor, and something else), as well as use various hours-long buffs (Hero's Feast, Greater Magic Weapon, Magic Vestments, Superior Resistance). He has a Rod of Chaining.

A level 14 Druid/Planar Shepherd, whose class features are gutted so that he can't get any 10 rounds to 1 shenanigans. He's pretty satisfied with buffing up and smashing face. Chosen plane is Arborea, so probably nothing too abusable.

A ... rogue, I think, who's pretty content to stay invisible all the time and not do anything during combats. Has kept him alive so far, I guess. Indeterminate alignment, and probably won't matter much since he doesn't actually fight (has great out of combat skills though).

And that's probably it. The remaining two members are a Swift Hunter who dropped to a Symbol of Death last session, and a Frenzied Berserker that always tries to TPK the party, who fortunately died to said pit fiend last session and will soon be reincarnated as a Mind Flayer monk once we find his body (don't ask).

We will be getting a rest before we (probably) fight the pit fiend next session. We're not allowed to go shopping for magic items or scrolls, so we're pretty much left with what we have and the spells we can prepare, all sources open. The pit fiend's almost certainly going to have reinforcements, most likely more of the golems and steel predators we've seen throughout the tower, and potentially more creatures with class-levels as well. A Forbiddence effect hangs over the area, preventing teleportation over the area (so I can't even Psionic Teleport out if things turn south).

Any strategies for not getting slaughtered? Some protection or defense against Blasphemy would be awesome, but even then, the pit fiend's very dangerous by itself. And without Blasphemy-insurance, the wizard, if he's here, would be knocked out in a single hit while the Cleric and Druid would potentially be Str-damaged to insignificance. I doubt I can take on a CR 20 with this character alone, especially not if it has helpers (not to mention its 1/year Wish). Would really love some strategies or spell suggestions or whatever (especially cleric persist targets).

tl;dr: Pit fiend about to Blasphemy us to death. Advice?