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 !