Grinder is the third Guardian in Blasterball 3, seen on Stage 30. It is a large green orb (but its color changes as it loses health), which is half covered by metal armor. Fortunately, the armored side faces away from the paddle, but Grinder also has two long legs attached to the walls of the stage, equipped with roller blades so it can climb up or down in order to make it harder for the ball to hit its weak spot.
Blasterball 3 - Stage 30