The 14 hit combo:
X-X-X-Y-X-Y-A-Y-X-Y-B then A-Y-X or A-X-Y or A-X-B

Take note that any of these things will end the combo:

If the enemy hits a wall.
If the enemy hits the edge of the arena.
The enemy blocks your attack.
The enemy hits you

This will be done on any of the bosses. Also, make sure that you attempt this on one of the earlier bosses, as the later bosses will end your combo.
As the enemy attacks you, deflect the attack, and this is where you want to start the combo.

The first part of the combo is the easiest, with the Y-X-Y-B, but after you will throw them in the air using B. After you have them in the air, you will then press A again to jump after them, once you do that, you press Y to do the magic spell attack on them. And finally, you press X to issue the final attack.