Kendo is easily the simplest sport on this (so called) game.
To win in kendo with a single strike, you need to be in a 1-strike match. Then you just need to hit the opponent.
Now this game is pretty ridiculous, so it most likely WON'T pick up your motion on the first try... so go crazy and just swing like that over and over and over and over. Also, the opponent can evade, so there's a chance for that too. Just keep swinging until it happens.
This can be done on any difficulty in any game-type (Xbox Live has not been confirmed, but online is dead anyways). if you can convince a friend to join in and let you wail on him, that would be ideal. But if not, the Local League/Cup should suffice.
Good Luck (: