Alien Zombie Megadeath

There are a total of 71 levels in the game. You need 180 medals to unlock them all. Unless you really pick things up quick, you'll start to hit a wall around the 100-120 medal mark and will need to go back to earlier levels and work on some of the previous medals you weren't good enough to get at the time.

Since medals will unlock even if you don't complete a level, certain types of medals are easier than others in the beginning levels, particularly the UFO ones. If you're hard up for a few extra medals and still have some UFO ones that are locked, try to knock them out, as they are generally the easiest type of medal to get. See the All the Medals trophy for gameplay tips and strategies.