Atomos does nothing but repeatedly cast Meteor but it won't deal lethal damage unless the party are under-leveled. It is also a good idea to cast Float on the Party to make the possibility of it casting Earthquake a non-issue.
Casting Slow at the outset will increase the time between Meteor spells, while casting Shell on the party will reduce the amount of damage taken from Meteor. It is helpful to have Haste cast on the White Mage; using multiple White Mages can also be helpful.
Atomos is the Greek word meaning "indivisible" and is the the origin of the word "atom" which means "that which cannot be cut."