Atamos has high HP and and his rather nasty comet attack, but thats it. just hit him hard and fast with the three players you have left after hes killed the first 1. i cant remember what level i was on but i walked away with all my characters alive.

i was told by an american friend that casting blue magic on him works. cast black shock and then level 5 doom and hes toast. i havnt tested this tho so if you do, let me kno if it works