I found this achievement rather difficult. However, I have found out that you can minimize your suffering for this achievement by playing a local game, with 7 easy-difficulty CPU players. Set the map to "Giant", the game mode to "Battle", the time limit to "9:00", and the rounds to 9. Set the only items in the game to "Speed Up", "Bomb Up", "Max Fire", "Dangerous Bomb". Make sure the items CANNOT be destroyed.
It will take a few tries, unless you're lucky, but the tip is to get the "Max Fire" and "Dangerous Bomb" power ups as soon as possible. Then you can start destroying many soft blocks at once, and hopefully kill 3 CPU players as close to their spawn points as possible. You may also be able to use this method on a smaller map, but you may have to take more attempts, because the computer players may pick up the items first.