When you first visit Nene's castle, you will come across red tripwires that, when tripped, will result in an enemy encounter. After you fight the boss in the castle, you will come across a long room with a treasure chest within. In the very beginning of the room, there is one of the aforementioned red tripwires which will result in an enemy encounter if tripped. If you use Field Barrier here, you can run back and forth all the way down the hall and back to the tripwire, you can spawn a new enemy encounter each time, which is an incredibly easy way to earn SP.
