It was only one attack, number one, and number two I know that even as powerful as it was that it couldn't destroy an entire cloud of heartless (like I said, small chunk possibly). It was also Ultiga is the only spell he actually used to attack the heartless, the Light Mega Flare is what he put into his Keyblade (to allow his attacks to be more effective by imbuing his sword with light essentially), Ultiga having essentially took out a bunch of his Mana and requiring him to fight with his magic imbued blade after that. 

Not only that, but the Ultiga didn't flat out destroy the magic resistant heartless. 

And yes, you are correct that Oner does get somewhat tired after firing off Ultiga (or Reno for Meteoga if he were to use that). which is why they will probably still stick to their lower tier counterparts Meteor (for Reno) or Ultima (for Oner) a little more often since those don't use as much energy or Mana (those two are not like the elemental spells they possess, for them it still matters which tier they use unlike the elemental ones).