Taken from http://www.xbox360achievements.org/game/xevious/guide/
Ground based enemies are the enemies that you destroy with a bomb (to use a bomb press B). You can take out two enemies with one bomb by aiming right in between them if they're next to each other. Not much help i can offer on this one. Try to stay alive as long as possible and keep bombing. Note: This achievement doesn't stack and must be done in one game.