I found this easiest to do while you're on the round platform fighting Cleopatra. As long as you do not attack her, she will keep sending 3 unbaptized babies at you. As far as I could tell, this is infinite.
Simply wait for her to release 3 babies, retreat to the far side of the platform, counter using LT, then attack with Y. You will know the counter worked when the game slows down while blocking the baby's attack. Using the Y attack will usually kill all 3 at once, allowing you to get to 20 quicker.