Is there a way to fuse Tonberry or Don Tonberry with an area of effect attack? For example, Ravage or Blitz?
Unfortunately, the infusable abilities for Commandos like Tonberry and Don Tonberry are slim pickings:
Of those abilities, only Ruinga is area-of-effect. You can get it from a number of high-level sources: Sakura Yule's Monster FAQ is a great resource for finding out which monsters get which abilities for the purposes of infusion.