Repeating what I said a few weeks ago, I love this game. 5 on 5 squad battles, big variety of weaponry, vehicles, and several maps (including one with roaming zombies). Each mission lasts only a few minutes, so failure is just another reason to try again. Free, and advertisements are entirely optional. You can advance to better weapons quicker if you spend money, but I've seen no need to.
Available at iTunes.