I'm not going to lie, I hate Blasphemy. It's way more powerful and useful in the hands of NPC enemies than it or the Holy version can ever be in the hands of PCs, since enemies are almost always several HD ahead of the PCs. The worst part is, it's almost like an auto-win button. Sure it's a high level spell, but if the enemy has several castings of it, then BAM, you lose a turn. Do not pass go, do not collect $200.
I'm just venting, here. I'm fairly certain there's no way for us to actually kill the demons. They're more powerful than any level 20 PC can be without shenanigans. And they're not even the enemy we were sent here to fight. If this is what we're up against first, I can't even imagine how the real enemy has been modified to make it tougher, because these guys could smoke the real threat as-printed based on what we've found out. No question.