Bullying is remarkably complicated.
"Just walk away" Is not an option. Part of bullying is that they trap you, they corner you, they box you in. you CAN'T eject.
"Stand up and fight back." Only works if you actually win the fight. If you lose the fight the bullying persists.
You could stand up and fight back and fight dirty. Gouge his eyes, kick him in the balls, do whatever you gotta do to end it. But then you incite an arms race. You punch him with a lock, he pulls a knife... etc etc until somebody is in the hospital with life altering injuries. Maybe him, maybe you.
EVEN IF YOU WIN THE FIGHT, other bullies who didn't witness it, or ones who think they prove something by beating you are going to pick on you.
Bullying was so bad for me that I was becoming suicidal. Beatings and encounters were escalating to near lethal levels. I gave up hope I was going to survive. I remember one point 4 guys were knocking me down and spitting on me when the unthinkable happened. Two other students stepped in and stopped it. They shouted, they argued, I ran like fuck so I don't know what was said.
The next day those bullies were waiting for me outside the school. They chased me down. Cornered me. Pinned me down when I tried to run. Then apologized. They told me we were going to be friends from now on. I was too terrified to argue. I was pretty certain it was some elaborate revenge prank or something. But they were legit, and turned into some of my closest high school friends. It wasn't enough to stop the bullying, but it was enough to pull me back from thoughts of suicide.
It wasn't the bullying that was killing me. It was the sense of abandonment from my peers. The instances where my victimization would draw crowds of gawkers, but later when questioned none of them had seen a thing.
It took time to recover myself, but after I did I found couldn't let myself be a bystander. If someone else was being bullied I couldn't just keep my head down and be thankful it wasn't me any more. I started stepping in. If someone had to be bullied in my school, then it might was well be me. I would be the lightning rod.
Yeah it meant I was bullied more. But some of those people needed someone to step in as much as I did. Before I knew it there was a growing number of people who had my back no matter what.
There is very little a victim can do to stop bullying. But a bystander who refuses to be a bystander can change a world.