I like all the mechanics used by just shooting, however, if you simply fly backwards the entire time you'll never loose, you'll be hit by a stray square or two from behind, but other than that you'll have a large mass of squares following you because they move slower than you. Even with the stray squares that will pop up behind you as you run into them, they aren't nearly enough to counter your fast health regen. Nice concept, just needs some balancing to encourage the player to fight more.
Suggestions:
1. make a successful kill worth points (right now the game just gives points by how long you survive)
2. Make the blocks faster
Or
2. Make more blocks spawn
3. Make the shots bigger