The easiest method of doing this would be versus mode, preferably with a second controller. Regardless, go in to the fight after selecting Ragna 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 grab the opponent from the ground and stab them with your sword. Simply repeat the process three times and you should have your trophy.