Dragon Isles; getting clever about the bodyguards is good, or being able to spike them while leaving a lot of popcorn up top to burn off all the Curse layers.
There are a few good approaches here.
One is to worship the Glowing Guardian. The "Enlightenment" boon removes all curse layers, and is the only effect in the game to do so.
Another option is to use the LEMMISI and WEYTWUT glyphs together. This allows you to reveal the Matron's bodyguards, then swap positions with them to get to the Matron without fighting them. You can also use the ENDISWAL glyph to bypass the first two guards. As Wargasm mentions, WONAFYT is another option here.
Then there's the simple approach of not bothering with resistances. Curses can't hurt you if you don't have any resistances in the first place. Use heavy-hitters like the Rogue or Warlord for this approach.
Since Wargasm doesn't cover them, I'll talk about the other two vicious scenariosNamtar's Lair
This level can be tough to get traction in. I find Jehora Jeheyu is your best deity, since his health boost enables you to survive the boss's high attack damage sooner, his weakening/corrosion clear helps remove the debuffs the boss incurs, and his boost mana boon effectively cancels the crippling penalties you suffer as you approach the end of the dungeon run.
The biggest thing to beware of is that one of Namtar's later forms has 75% physical resistance. Since you'll be cut off from deities at this point and carrying a RBS this deep into the dungeon is impractical, you must
take BURNDAYRAZ with you and have some method of mana spiking (usually BLUDTUPOWA or potions).
I personally recommend the Warlord for this dungeon.Demonic Library
The key here is to begin the boss fights against the demons as early as possible. Their stats aren't actually that impressive, and some of the level 8 and 9 monsters are actually stronger than they are. This means that they're great sources of bonus XP. Because you will have trouble surviving their attacks, so glass-cannon style characters like the Warlord or Rogue work very well here. Be mindful of the demons who have abilities that are strong against you; you want to leverage level-up catapults to weaken them before making a swift kill.