Also in addition to Crimson Drifter's solution BUT do this on the Southwestern Deserted Island (Poo Island).
When a Platinum Poo Snake appears press the X button to turn off the Field Barrier, then use the RT button to select it and one other enemy for a Multi-Monster fight.
In battle, kill the first group of enemies BUT when fighting the Platinum Poo Snake, keep defending and allow it to escape.
Once the battle is finished you should recieve around 40+ SP. With Black belts and the Eyepatch equiped that number will be double, meaning you can get 80+ SP for just one fight.
I discovered this exploit completely be accident but has worked every single time I have tried it.