You need to undo his transformation before you will be able to fight him (if you have not completed everything else in Broken Valley, another level or two wouldn't hurt).
Explore everywhere (in the chamber with the bedrooms upstairs); essentially, you need to find the book the librarian is looking for in order to get the associated volume, which (with a couple ingredients) will let you turn Amdusias human.Help on Killing Amdusias. Any tips would be nice.
BTW, after completing Maxos temple, you will have access to Sentinel Island. Your first encounter there you can mindread to go up a level, so the higher level you are (I made it to level 18 in the fjords) the more of an experience bonus that will be. You don't need to get to a high level, but it would be better if you got to a new level before entering, to get a full level bonus.
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