This achievement isn't too hard, but it can be depending on some uncontrollable factors. You have to win a player match online.
If you don't have much Tekken skill, and you're just looking for an easy way to win matches without trying very hard, just play as Katarina and spam the B button while not moving the left stick. This unleashes a flurry of kicks, but not everybody online will be dumb and not block anything.
You may have to fight a couple different people before you get lucky and face an opponent who is very bad.