At about halfway of Chapter 2, Fia will tell you that you will now be able to do practice battles. When she says that, press Select and choose "Practice". You will be able to choose an enemy to face then start the battle as normal. Choose items and weapons which your characters still need to get experience for, and max it out to get stronger. Also, the items and weapons you use in a practice battle will not get expired. It is just like a free battle. Unfortunately you will not get any TP points, but will still get new items afterwards.