You must have the D-Glaive Weapon for Zero, the Rasensetsu (the weapon obtained from Dark Mantis). Enter a level with Zero and look for a the enemy that shoots three rockets at you at the same time. Equip the D-Glaive and stand far enough from the enemy to where you will not kill him, but will still be able to get him to shoot rockets at you. Then, perform the spin attack obtained from Dark Mantis in mid air. After awhile you will rack up a large number of combo attacks.