I don't think it is about strength or power.
I think the problem is that Atum is made for a different kind of enemy.
He seems to be geared toward mystical foes. If Exitar was mystical in nature, Atum would probably have the silver bullet to beat him.
But, he is not made for cosmic combatants.
He can probably be a danger to mystical beings that are equal to Exitar's power level.