Tell him Natsu's a better fighter than him.
Super glue his clothes onto him so the next time he goes to remove his clothes he can't get them off.
Blame him for his master's death.
Tell him, Lyon was a much better student to his master than he was.
Disrespect him and his friends.
Spray paint his hair pink.
Put make up on him while he's sleep.
Pour ice down his back.
Challenge him to a fight but then don't show up keeping him waiting all day.