If you want unlimited action points do the following:
Select a mission and four characters. Equip and deploy them and when 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). Go back to the playing screen and you'll have unlimted action points.