I had never tried the Demonic Library before today, but I gave it a shot and finished with a Trisword Dwarf Rogue, which I hear is a pretty good combo.
I enjoyed the whole dungeon and found it to be a comparable challenge to the Dragon Isles, which is the other vicious dungeon I have completed. At first I wasn't worried about the mini-bosses... then I realized that the Physical Resist, Weakening miniboss was getting death protections... and I had converted my BURNDAYRAZ glyph. Oops! I had enough resources to finish him off and make it through, though.
At first as I was looking at the bad guys that are summoned, I was like "oh no! they're going to cut me off from upstairs!" Then I realized that some are significantly weaker than others, and that anyone with access to first strike can treat the weaker no-physical-resist ones as popcorn.