Valkyria Revolution

Free missions are optional missions with blue icons that you can choose from the map. You will likely do a decent amount of these during the story to help level up, but otherwise you can grind out the rest after the story is over if you need to. The quickest ones are the Anti-Valkyria Squad Training, and anything where you need to kill a specific enemy, as you can just run past everyone else and kill them to end the mission. Sometimes no Free missions are available (only Battle missions), so you'll need to do one of those to get a new set of missions.