Although I'm a little gal, so I couldn't do a whole lot of good anyway. The best I could hope to do is a short physical attack that allowed me just enough time to escape-don't know that that would help another person.
That's a sensible attitude to have, but even a short attack can have devastating effects. When I lived in Mexico City back in the '70s self defence skills were needed nearly every day and one of the first things I learned from the Bruce Tegner books was the hop out the way and stamp on the side of the knee. Highly effective and little effort required.
I soon found I could adapt it to help other sentient beings (usually animals) being attacked by groups by 'duck hunting'. That is moving in behind the group, and stamping on the top of the calm of the rearmost one, grab him under the jaw to stop any noise and the hair on the top of the head at the same time. Step back, rotating his head until his head nose was uppermost. Then move to the side at the same time releasing the hair and placing that hand on his forehead. Then 'fire' his head down at the pavement by straightening my arms in one move -like bouncing a basketball. Then move on and repeat until the rest of the group became aware of me
I once 'took out' four of a six strong gang before the last two realized what had happened, they decided the odds were not in their favour and ran off!
There are also martial arts such as Wing Chung which are designed around science rather than strength that are reasonably quick to learn and highly effective.