*NOTE: You'll need the airship for this trick! First, you'll have to get "Capture" weapons from the Monster Arena guy in the Calm Lands. You'll also have to catch all the monsters in the Calm Lands in order to be able to fight the monsters you catch in other areas. So, when you finish catching all of the monsters in the Calm Lands and the "Fight Monsters" option appears when you talk to the guy, go to the Omega Ruins (X: 72 Y: 34 on the "Search" option in the airship menu) and catch a Master Tonberry. Once he's caught, go fight him in the Monster Arena for 30,000 AP per battle. If you have Double AP or Triple AP on your weapons, you can get 60,000 or even 90,000 AP in one battle!! If you continuously do this for a while, you can get some SERIOUS Sphere Levels. I did this and have about 30,000 HP on all of my characters, along with stats in the 200's.