What I did was fight on the veldt with gau and learned all the spells I could without gaining EXP. Then I went to the forest with T-rex battles and leveled up as much as I could. If you put in 1 person with the EXP egg and the gem box, you can level up every battle. Have the character learn vanish doom and cast it on the dinosuars in that order. Quit when your not leveling up every battle and you should be on a level good enough to fight the final dungeon.

If you don't want to do the Veldt then the cactur desert is better for AP.