Yes, I commend you for trying, but level 11 is really far too low to be fighting Amdusias, he is a pretty tough boss. Things after him are level 16+, and don't get any easier. I recommend reaching at least level 15 first before fighting him.
I killed Amdusias at level 14. Used Stun Arrows first and after that Rush Attacked him so he MUST start with his sword. My Summoned Ghost heals me. My Creature had a Mage Head (with Magic Blast & Magic Missile), but with a Warrior Head (with Fatality & Rush Atack) it's probably better !
Or you can try to use Hide in Shadows, run to Amdusias and start with a melee weapon. (I'm not sure if Amdusias is immune for being stunned).
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 !
I used a duel weild swords & to slap him around up close & when I needed to back off to heal I hurled a few magic missiles his way. Magic Missile with 5 points in it is very effective & kills him very fast