Bomberman Battlefest

Map: Classic
Mode: Battle
Players: 2 Human Controlled Players
Time: 9 Minutes (Not completely necessary, just in case)
Powerups: Bomb-Up, Remote Bomb

This is the easiest method:
Destroy blocks until each player can drop at least 6 bombs, and has the remote bomb powerup.
Then simply create a row of 12 bombs and move both players to a safe location.
Finally just press the 'B' button using the player you want to unlock the achievement with.

Using remote bombs makes it easier because it essentially gives you an unlimited time to set up the 12 bombs, rather than risk killing yourself in the process.