I'm not sure if it's accurate since it was quite long ago when I defeated him, try it though.

Kenji's strength is his reflexes, it's VERY difficult to smash the ball past him. What you need to do is back-spin shots. Just keep applying back-spin. He usually won't be able to smash the ball and he'll make mistakes a lot. If he missed completely, good for you! If he returns it close to the net, look for the opportunity to smash cross-table. If he returns it close to you, apply more back-spin. If he manages to smash it, DON'T attempt to put spin on the ball, just play a safe return.

That and a little bit of practice should do it