Using the Cardinal's boomerang, face the catfish boss. Find a spot where you aim at it 
while it rises and charge your boomerang. Release it as the boss rises so it becomes 
stuck on his back. Attack it continously to get a good amount of exp. Repeat as needed. 
There is a risk of a glitch that will happen while doing this trick. 
Return to the dashboard and launch the game again to fix.