To get this achievement as easily as possible, set up a custom game with the following settings:
Game Mode: Deathmatch
Game Mode Settings:
Score to win: 50
Advanced Game Mode Settings:
Self-Kill Penalty: 0
Respawn Delay: 00:00
Bot settings to: Fill Empty Slots and Easy.
Confirm setting with the Start button
immediately when the game starts, select the Chefolet and aim down the middle (Blue/Purple) hallway. Fully charge a shot and have it ricochet around the arena. With a little luck you'll hit more than 4 people with one shot. (Also, if you happen to hit yourself, your death will also count towards the achievement.)
Also, this setup can also be used to accelerate the exp earned to hit Level 25/50.