An Overdrive Blast is the super weapon you can use after scoring multiple kills. It goes down if you aren't shooting anything. Once it hits 100%, you just tap the bar on the top of the screen. Alternatively you can swipe 3 fingers upwards to activate it. A blast goes from bottom to top killing everything in it's way (except bosses). You will likely just get this playing along. The farther you get in waves, the higher chance you will get to killing 50 in a single shot. The achievement unlocks after the game ends.