I for one was lvl 13 when I could encounter Amdusias, but it was too difficult, always one-shot me with his magic.
So I decided to finish some side quest n kill some things I didn't kill yet in Broken Valley.
Got me to lvl 15 as I was hoping for. Got me the very powerful boost on magic blast (lvl4), got me hide in shadows and fire wall lvl 1.
Killed Amdusias without getting hit :o, magic blast (stuns him a bit), jump backward(so I'm out of "sight") n wait a bit, hide, run forward next to him, wait a bit(cooldown magic blast leftovers), firewall and magic blast, that did it ^^.