Once you kill the final boss you can continue to use the battlefields and level up all you want. The "Enter Battlefield" option will become active on your menu.
The exp in the after game battlefields is very high, and the monsters get very tough to beat.
A word of caution: When you enter the after-game battlefield for the first time immediately save the game, then reload your save again. If you do not do this you will run into some major problems with crashing as you train to higher levels.
