I use also high potions (Ultmiate ...) of Resistance, Strength, Dexterity, Intelligence to be stronger!!
I did this final battle in totally with maximum 5 great healingpotions and the skills Magic Blast, Fire Wall, Destruction and Confusion and futhermore a great high one handed sword and a good shield ... (the weapons of the kind: Legendary or more better: "Unique"! = red color!)

I was level 35,5 on that moment.

Also I summon my creature to help and (with Crystal Skull). In Fire Wall I had 7 points and also in Magic Blast 7 points I believe.
Make sure to jump away (if you can) when 2 or 3 surrounded you! Or use Wirlwind!

I also concentrate me first to defeat the one who can heal others!

On 7th of february 2015 : I start a new adventure in the Divinity world of Original Sin,
it's a Fantastic Freaking Fabulous Funny ... it's my All Time Favorite One !