Easy Way To Level Up:

Go to the forest of death. travel all the way to where you fought the three rain ninja. In that area are two battles that give you lots of exp. One of them is a battle with three chain centipedes. You get 5400 exp total and if you have three ninja they each get 1800 exp. The other battle is with two chain centipedes and one rock toad boy. This battle gives you 4077 exp total and if you have three ninja they each get 1359 exp. There is also a leaf symbol to the north in case you lose health or chakra.