That would be the final part of the N-Zone, in the crystalline area right before the final battle. The monsters give about 30 ABP every battle, though they don't give XP.

Don't fight King Behemoths by the way, unless you like getting slapped with Meteor every time you hit them.

Uncommonly, you might encounter three Movers in a battle. You have to defeat them quickly, or else the battle automatically ends. Use attacks that target all enemies. They are weak against Fire I think.