*Equip the 'The Comet' blade at Sensei's Swag (Dojo > Sensei's Swag > Space Capsule > The Comet)*
At the end of Arcade mode a reddish glowing fruit appears. This is the one you need. Slice it 40 times for the achievement. A good way to do this is to make small and quick slices with both of your hands. You don't need to aim, just slice away. Achievement unlocked!
The pomegranate sometimes appears in Classic mode too.