Hey guys, I have been trying this achievement for hours until I tried out this method and it allowed me to pretty much perfect everyone apart from the 2nd fight.
Difficulty: Hard (obviously)
The method only requires you to do one move during the fight, so here we go.
As the match starts run backwards or in whichever direction to get yourself to the edge of the arena. Now the enemy will try and catch up and hit you, at this point you want to use Down+Right+X (Hold). This move will create a guard impact for a split second, blocking the attacker and flinging them over causing a ring out.
The reason I said this won't work in the 2nd fight is due to the arena having no ring out function. In this case you want to use the moves you know best and just KO normally.
The reason I find this method better is due to the fact that the computer on hard likes duck or grapple break you.
Hopefully this helps you guys as much as it helped me.