challenge?"
1) enemies that you don't kill before reaching the bottom of the screen will re-spawn at the top and move twice as fast as they originally did. they will continue to re-spawn (although they won't get faster) over and over until you do kill them.
2) each time you collect a new crate, you discard your current weapon for the one in the crate.
1) run to the next crate as quickly as possible so that you can get a new weapon, all the time hoping it's better than the one you have and that only a few enemies manage to re-spawn during this time.
2) attempt to kill the enemies with a 