Most credit goes to x360a for the main tip, and that is to play Floor 11 on Tower Of Lost Souls mode, because you have a high defense, and are up against 4 characters with high defense too.
To add my own input to this solution, I used Voldo, but the following should work for any character:
For the head armour, use X to hit high. Works best following a hold joystick in a certain direction + X attack.
For the torso armour, use a running attack followed by the X button. A few of these should see the armour go.
Finally, for the lower armour. Use an attack while holding down, either the X or Y button, and hope your opponent blocks low.