Arrrggh! The last level is really doing a number on us. I have reached level 31 and unlocked most of the required upgrades, along with max cool-down and increased radius of my air-strike. However, the amount of enemies spawning towards the end is insane. You'll practically have wave after wave of tanks and smaller buggies, until you finally get the boss battle, which is another challenge. Our first attempt this morning, ended badly, with my brother hitting 'Game over' right in the first wave of attack. I stayed until the final boss battle, only to be over-powered later on. Will attempt it again tonight and seal the deal.