Select a mission and any four characters. Equip and deploy them and when you're on the main screen, click on the blueprint icon on the menu. Select each character in the following order: 3, 4, 1, 2, 2, 4, 1, 3, and hit the left hand icon (the rewind button). Return to the playing screen and you'll have unlimited action points and be able to skip to the appropriate level by pressing '1' to '5'. Once you've skipped or completed a level you'll have to repeat this process.