[Rancour]

I started a new game with a multi class character - elf,fighter/mage/thief.The idea was to complete the game with this single guy.But as I have already passed SoA three times,and I wanted something new,that's why I rushed to the Watcher's Keep immediately after leaving the Irenicus dungeon.The only thing I've done was calling at the Copper Coronet,buying a nice blade of roses,completing that dull corny quest about slavers.
Completing the first level of the keep on the 'insane' difficulty was rather challenging,but I managed to beat it.
On the second level,I killed all the monsters,took all of the scepters,but beating the demon is impossible for me.My current level is 9/11/11,I dual wield foebane +3 and the blade of roses +3.The problem is,the beast regenerates too fast,and there's nothing I can deal much damage to it with.All of those elemental damage spells(firebolt,ice storm,cone of ice,lightning) deal about ~20 damage each,and the demon kills me really quick as soon as the stoneskin is gone.
I don't want to give up and try to earn some experience elsewhere.
Any suggestions how to kill that damn demon?