This achievement is for using the Reflective Disk weapon against any other weapon and killing your Opponent.
-Set up a Death Match game, use Slums at the bottom
-Set the starting weapon to normal shot
-Turn off Power Ups except for Reflective Disk(they will be red if they are off). This will allow you to pick up the weapon without your enemy having it.
-Set the bullet Speed to 25% to make this REALLY easy
-Set the limit of kills to max value and timeout to none
Line your player up against the enemy. Shoot from the enemy and immediately shoot back with your player and reflect back their shot to kill them.