I defeated it with a Sorcerer. If you are talking about the Curious variant, the final boss is just WAY easier with the Sorc.
One important thing here: You have to defeat BlahBlah twice, so it sucks that he blinks all over the place. The solution: Prep Knockback and go Binlor. As long as you knock him onto an empty tile/a wall every single attack, he will not blink, which allows you to kill him way before exploring everything. Thus, you have enough black space to regenfight the last boss. Oh and definitely use Binlor to have maxed out magic resist it help a TON against all bosses here.
Meanwhile, I am trying to tackle Demonic Library but I got nothing... Unstable on every single enemy is just horrible

Any ideas here?
EDIT: Nevermind got it! Gorgon was really strong here

Had tons of ressources left at the end! I just loved all Curious Vanilla Vicious dungeons. I mean wow, there were just small changes, but they made the dungeons really interesting again!
For difficulty, I would rate them from easiest to hardest like this:
Naga City - Dragon Isles - Demonic Library - Vicious Halls of Steel - Namtars Lair
Now to Heretic Gold! And then the curious extreme vicious dungeons will fall...
