I thought I should add this solution as it's way faster than the one above and doesn't require you to have any skills unlocked.
What to do is get to the School Invasion Stage and get to the bit where you have to get the bridge up to prevent enemies from getting in, but don't rise the bridge keep it down that way there's unlimited baddies.
Now all your need to do is press B to do a kick to them then quickly press and hold LT for focus and press and hold X till there's a shimmering light coming off the tip of your sword and release don't add any direction to this attack and you should be getting the heads chopped off quite easily and fast.
Press LS and RS down to see your progress for this.
That's it, I hope that helps :)