Here's an easy way to level-up. When you return to Abrabah for the second time, complete the ruins stage. Afterwards, you can return to the ruins area and fight an unlimited number of enemies there as many times as you desire. If you enter the ruined tower and access the minigame within (by pressing the switch at the back of the chamber), you can fight and kill most of the enemies with one hit. Rapid Thrusters will give you 47 experience points, Hook Bats will give you 117 experience points, Fortune Tellers will give you 350 experience points, and Crimson Jazz will give you 585 experience points. One run through the minigame (2-3 minutes spent) can net you as much as 5,000 experience points. You can repeat the process until you reach the desired level.