Use A and D to go left/right, and use S while in the air to do a downstab thing. If you've played shovel knight, you know how it works. But you can't do the normal shovel swipe. You have to wait for him to charge through the fire things to attack him. If you touch the flame thingys you will go to the center
I stole stuff from Super_Scratch_Bros20 but I edited some of it. It's from his project "King Knight AI". Go check it out.