You can one hit KO the final boss by switching weapons and attacking before you've fully swapped weapons.

Switch weapons > Attack as FAST as possible mid switch = OHKO

Your weapon will disappear from your hand mid-switch, and must confuse the game into applying ridiculous damage to the boss for some reason. Maybe it doesn't know what weapon you have so it just breaks the fight, we really don't know why this happens as of posting this.

As far as I've tested, this only works on the final boss. This can be done at any time during the final boss fight with Jin and Mugen both.