I had a fight years ago. Guy was swearing at my classmates and wanted to fight so I tried to stop him but he turned towards me and tried to head butt me but I backed off and started punching him in the face. He fell down then I put his shirt over his face and continued punching him. When he got up his whole face was changed and I instantly regretted of what I did so I ran away.
First check if his hands are empty. If he is holding anything then run away. If he is bigger than you(not the same weight class) run away. If you think you got enough punching power to knock him out then start throwing punches and only target the face. When he is on the floor knocked out cold then run away.