I was level 13 with 4 skill points in magic missile, I have also used ranged weapons to take amdusias down. There is a slight hill in the bridge that leads to his blood pool, I hid on the ridge and sniped / shot missiles, and whenever he would fire those exploding arrows, I'd duck back over the ridge...

"All that is necessary for the triumph of evil is that good men stand by and do nothing" - Edmund Burke