This is what I do when I defeat Kerpymon. Keep on hitting his head.When he appears in the middle of the screen, it means that he is going to use Black Hole Attack. Run to the corner of the sreen so he won't get you. After using Black Hole Attack, he will create a Tunder Cloud. The Tunder Cloud will make tunders to attack you. Run(not walk,in case you don't know,press the direction twice quickly to run) to one side of the screen and quickly jump to the highest stone and then attack the cloud. After you do this,the cloud will make tunder to attack you immediately. Dogde its attack and run to the other side of the screen.Do the same thing until the cloud disappears.When Kerpymon is about to "die", it will turn red.