Make another object the same size as your character. Match up the new object and your character in the editor. Pin that object to your character. Make the object invisible. Change the objects collision polygons so that it matches up with your characters feet. Make sure that you leave a pixel or two on each side, you might need to play around a little to get it exactly how you want it. Set up that object in your events so it does damage to enemies, rather then your character. Also, you may need to update the collision polygons on your character so you aren't also taking damage when you jump on the enemy.
There are probably better ways to do it but that will work and get you started in the right direction.