The easiest method of doing this would be versus mode, preferably with a second controller. Regardless, go in to the fight after selecting Noel and press 3+C (Look at the number pad on your keyboard, it will be down/toward if you've started on the left + Circle.) This should place your foe on the ground, now you must press down twice and C and you will shoot the opponent while they're down. You can keep doing this even if the combo goes blue and each hit will count. This must be done 10 times in one match, though it's easily done in one round if done correctly.