Instructions
Follow your map and head to the Northern mountains before you have unlocked Goku. Here you will find a barrier Goku is supposed to break. On the other side of the fence is a dinosaur; you can hit the dinosaur through the fence and gain a lot of experience in the process. Repeat this tactic to gain lots of experience and level up quickly.
Return to the Ninja Castle where you fought General Tao during your quest to find the Dragonballs. Ninjas will continue to appear in the area so just keep fighting them to collect the experience points. While you will not obtain a huge amount of experience points, you will consistently receive points as the enemies continue to re-spawn in this area.
Head to the Snow Mountains and look for a black mark in the upper, left corner. Punch the wall here where it appears to be slightly cracked to reveal a dinosaur. Defeat it and claim the experience. Defeat the monsters outside and return to the area and repeat the process for more easy experience.
Unlock the passage to the Warlord's Domain. Visit the thief's house, here you will find a punch bag and every time you punch it you will receive one experience point. Continue to punch the bag for lots of easy experience. This technique is best used during the lower levels.
Repeat the process with all of your characters to easily reach Level 50.