The above solution works too, but if you want to make it even quicker and easier, when you use Kenny in a combat, have him (her?) use his second ability, Furry Friends. If you don't do the "shaking off" it shows, this move will still damage the enemy, but will also kill Kenny. What I'll typically do is save it until there's only one enemy left, use this move, not "shake them off", let Kenny die and finish the fight myself.
Edit: This also works for his third ability, Unicorn Stampede. Just don't hit the buttons that come up. Like the rats it will still hit your opponent, but kill Kenny in the process.