This is a truly excellent game that, aside from great reviews, doesn't seem to have gotten the exposure it deserves.
It starts out as a really solid twin-stick shooter, but as you move up the levels you start to see just how strategic it can be and the number of different tactics you could employ to get the job done. Just shooting stuff won't cut it at the higher levels, and the highest two difficulties require absolute coordination making them difficult to play with randoms.
Great game though, fun in general with a very strong challenge to boot for those that like strategy and difficulty.
Oh, just to add, it's essentially Starship Troopers: The Game.