That's because Amdusias is most weak in melee !
You must have had luck he had not hte change to shoot an exploding arrow at you ! And he can use also Fear or a Stunning Arrow ! But attack him as fast as you can with a melee weapon and you got him on his most weak spot
I use Hide in Shadows or Rush Attack to fast attack with melee weapon to force him to draw his sword ...
My Creature with a Ghoulish Head is very useful doing a Rush Attack and making an end on Amdusias' life with a Fatality blow !