This is actually pretty easy, just be sure you buy Unlimited Kokonoe in the Gallery, make a Vs. match with any character with a second controller and spam ↓↓ A, the key is spam the buttons as crazy, just spam the A and the ↓ button at the same time as fast as you can, the enemy will be stun forever in the air with a blue combo and 20 second later the achievement will be yours