It's been out for a few years now, but I only just today got around to playing
Atom Zombie Smasher. It's a pretty unique game. If you try to un-blend back into its genre pieces it's a tower defense-real-time strategy-choplifter game.
The basic idea is that you go around rescuing civilians from zombies. Your helicopter can only hold so many people at a time, and ferries its cargo off the map once it's loaded. Meanwhile zombies are swarming and eating everyone. You get various types of other units to try to stop the zombies with, including infantry, snipers, barricades, and artillery.
It was a lot of fun, but I don't have the patience to sit down and grind out some of these achievements. For the two hours of gameplay at the 5 dollars for a whole Humble Bundle price I got it at? Yeah, it was worth it.