First you must transform Amdusias to human form by reading the spell you find in the nearby rooms, attacking him whilst in dragon form is futile.
It has to be an all out attack, trying to hide will get you nowhere, once you are in his lair the doors will slam shut and will not open until he is defeated.
Arm yourself with potions and have the low health notification set fairly high, run at him and attack with melee weapon as the bow is useless - a high stat 2h weapon seems better than 1h + sheild . He will use 'stun' arrows at first, the second or third arrow will kill you so get fairly close using the 'dip' in the skeletal structure . First, run and attack him, he will then use the curse spell making you run hay-wire and in opposite direction, it will last for just a few seconds, be ready and immediately run toward and attack him continually (button mash) and stay close.
I must have tried more than fifty times (only had one health potion!) i used the 'sorcerer Arben's sword of destruction' and trapped him against the bones so he had no rearward movement.