There are multiple methods to do this.
Start a game with 2 human controlled players. Use the classic man and battle mode. Turn the time limit up to maximise the time you have if it goes wrong. Also change the powerups so you only have
Bomb-Up and Remote Bomb in play. Once the game starts, blow up most of the blocks in the middle so you have enough room to put 12 bombs in a row. Make sure you have the remote bomb powerup with both players, then create a line of 12 bombs. When this is set up, press with your player and this will unlock. Alternatively you can do this with your boosting partner.