Lol at the thought of punching the alien and doing damage. Go punch a tiger. Even one that is mortally wounded. Not happening. We can't hardly beat a 50 lbs dog. Have you ever punched a person? It's not like the movies. Unless you catch that sweet spot, most people will absorb quite a bit of punishment. Now imagine the hard armor of an alien. How far from it's face is the brain? Loads of factors.
The only thing even remotely feasible physically would be to lock one around the body from behind while it was mauling someone else, and if you had enough strength and adrenaline you might get it off it's feet for a second, but it would probably twist around so fast it wouldn't matter.
I could see 3 or 4 people maybe tripping it up for a second while it was ripping them to shreds, nothing more. That is very suspect too. It would be like trying to gang up ona gorilla, but worse.