HOw do I beat these bastards. They keep hammering me so hard with all their spells that I don't even have a chance to do anything. They even see through my invisibility spells. Not that it would matter if I did actually get some decent attacks on them. As soon as one of my characters gets near the stairs behind them a whole squadron of shadows comes running down. Please I need some help before I go insane from frustration.

Summon monsters to draw their fire whilst your characters hold back and use spells and missiles. Try casting spell reflection or immunity on your mages too. Before you close, have an invisible mage cast area affect spells such as fireball. Later use chain lightning. Cloudkill can be useful, if you aren't followed.

Just buy Baldurs shield it reflects their rays back onto them.
or if u have the cloak of mirroring use that(same effect).

Wizard's eye and some summoned monsters seem to work pretty good too. Also using ranged attacks while doing this. Can take a long time, but then it's better than not doing at all. If you have the cloak of cheats, it's definitely much easier though.