If you finish off Broken Valley and still have trouble with Amdusias, you can head to the fjords (using the mountain path to the far south west in Broken Valley) to level some more. In D2:ED I went to the fjords after Maxos temple, and made it to level 18 (in DKS I did some before and some after and got to level 22, thanks to changes in some opponents and experience, as well as increases in early quest experience, etc, for DKS).
If you run and jump at an angle, Amdusias will have a harder time hitting you (the path isn't really wide, but it isn't so narrow you have to run straight towards him, either). Having your creature and/or a summon active may help, as well, if he targets them rather than you. There is a high point in the path that if you stand behind at the right spot you will be able to target Amdusias, but his arrows will be blocked (including explosive arrows).
BTW when you finish off Amdusias you will get access to Sentinel Island. Your first encounter there you can mindread to go up a level, so the higher level you are 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 (equip anything you have with a Wisdom boost).