Tips On Killing The Cashier
Killing the Cashier is possible and actually easy as hell. Grab a gun, (most recommended is HF Laser Pistol), walk behind him and start shooting him.
Note: You have to be very close to him, so he's unable to hit you. After you kill the cashier, you unlock Mini-Vulcan, and will be available on the matches.
That's all. And remember, practice and patience makes perfect :)