I do not know what I would do. There is no gun or knife, so this does not look too bad. The first thing the comes to mind is being sure the elevator door does not close, and yelling for help. I do have a strong voice. The guy on the floor looks capable of defending himself, so I may intervene with the idea that, with a little help, the guy could take over. Especially if he called for help, I would be moved to intervene. I do not imagine I would attempt to harm the attacker, beyond a bit on the ear, unless he attempted to harm me. After all, just walking into the scene, I am not sure who is the bad guy, and taking out his eye with my thumb might be over reacting and something I would regret.
Young women who find themselves in these situations and know the young men who are attacking each other, often try to distract the men. This can mean being seductive. At my age, ripping off my blouse would just be shocking, but if it works, it might be worth a try?