Its rather easy for Archmagi to defeat it if you find the right way.
You'll need Cleansing Flame or Disperse Magic to cleanse its beneficial effects, especially the main threaten: its reflective skin which reflects damage. Just remember to watch its sustain. Dont hit it too hard when reflective skin is active, dispel comes first.
With Phase Door/Teleport/Phase Door Rune/Teleport Rune at hand, it shouldn't approach your character. Its only skill that does damage at a distance is Eye Beam, approx. 1500 points of fire or cold or lightning damage. Its nothing if you got high resistances against these types of damage. Resistances are not mandatory,though, cuz' you can easily create a damage shield absorbs 1000 or even more with Arcane Shield/Shielding/Aegis and you'll be safe when these shield is active even without high resistances.
Atamathon is nothing but a huge pile of marble to Archmagi.