Select a mission and four characters, any will do. Equip and deploy them and when on the main screen click on the blueprint icon on the menu.
On the right there are the faces of the four men you picked to play the game. Simply imagine they have been numbered 1 to 4 from top to bottom then click on them in the following order: 3, 4, 1, 2, 2, 4, 1, 3.
Now return to the game by clicking on the top left arrow. You will now have unlimited action moves for each player and you can skip to the appropriate level by pressing '1' to '5'. Once you have skipped or completed a level you will have to repeat this proccess.