Punch his face by clicking it. When his face is red you can wait until it cools down or click and cool him down faster but he regains HP. I gave a little boost to myself, but eh.. I don't actually care.
If you kill him completely and click him one more time random music plays. That music is credit to @pococikapusta. Btw I do know what it's called. There is a glitch where you can accidentally overkill him and his hp goes under 0 . So try to be slow when reaching 5 more hp left.
