1: Use the book titled "Dragon Lore" in Amdusias' room
-From the Crypt Shrine, travel down the south hallway, then climb the stairs
to the second floor and look for the book in the northwestern room.
2: Take the Phial of Blood from behind the false wall you just opened.
3: Complete the quest 'A Tale of Two Tomes'
4: Once you have both the Phial and the book, travel down the west hallway and
make a mad dash towards the Blood Altar. You will find the scales you need
just in front of it. Pick them up, then immediately access your inventory and
use the Orbis Arcesso (under the quest items tab).
5: Defeat Amdusias.
-His explosive arrow makes him very difficult to defeat at a range. You can
try engaging him in melee to force him to draw his sword, but it doesn't
always work. If he does draw it, you can safely back away from him without
having to deal with his lethal bow shots
Gamefaqs has a walkthrough
http://www.gamefaqs.com/pc/952260-divinity-ii-ego-draconis/faqs/58701.