Eqiup redx2 on a black mage, white mage, time mage or summoner and they can double cast any spell they have learned.

Combine this with the time mages spell "Quick" for devestating results

Doublecast - 'Meteor + Quick' + 'Meteor + Meteor' + 'Meteor + Meteor'

Or using a mime you can replace Meteor with Bahamut/Holy/Flare etc