To easily gain experience, get a Chocobo Wing, Chocobo Feather, and Chocobo Down a day. When you get the experience bonus icon, fight until your Chocobo says you will get 5x experience in the next battle. Equip the character you want to level with the Chocobo Wing, Chocobo Feather, and Chocobo Down, and set Alexander (the manual control version) as your summon. Go to Quick Battle, and fight a level 100 Exdeath. Set the ruleset as Chaos Judgment doing nothing for the battle but landing HP attacks. During the battle when he is within one hit kill range, try to land ranged Brave attacks on him until your Brave is at 9999. While doing this, do not forget to get Ex Cores. When you have max Brave and a full gauge, activate Alexander and Ex mode. Land one more HP attack and your Ex Burst. If done correctly, you should jump somewhere between 7 and 25 levels in a single battle.