Rush Attack is indeed a nice skill against Amdusias.
But if you can find/buy a yewellery item with the skill Hide in Shadows on it that is also very useful !
I used a Creature with a Ghoulish Head (skills: Rush Attack & Fatality). Further on summoned my Ghost to heal me (best thing is to have your Ghost behind you so the enemy do not attack the Ghost directly ... Both, summon Ghost or Undead hurls fireballs to the enemies, but the Ghost can Heal you too
! But it's important to upgrade summons to the most higest level you can so the summon has more change to survive for a short time
After Amdusias become human form I jumped & runned fast away from him to the closed gate using Hide in Shadows so I'm sure Amdusias do not follow me !
After that I wait till the cool down time of Hide in Shadows is over, drink some resistance or other potion (strength, dexterity, intelligence) ... make a new save game, summon my Creature & Ghost, start running to Amdusias while using Hide in Shadows. Then, when I'm close by him, my Creature with Ghoulish Head start to attack Amdusias with melee. Now it's time to start attacking him with a melee weapon !! And if needed drink a healing potion to be sure and in the meanwhile don't stop attacking Amdusias with a melee weapon, don't stop so he have no time to attack your Ghost (and so the Ghost have time and a chance to heal you if needed !
And if you have 1 point in Rush Attack you can also use it when you're close to Amdusias using Hide in Shadows.
But beware: if your manapool is empty Hide in Shadows stop working (it use mana per second) and you become visible again !But you can drink a potion or eat, you remain invisible !
This way the battle was fast over AND I killed Amdusias
But a good strategical plan was helpful !!
Note : you can also use the spell Healing (required level 15) and remain invisible while using Hide in Shadows
Good luck !