Teaching your child how to handle a bully?

I know a child who has been picked on within a small group of boys, and I don’t have much experience with adolescent boy interactions. The boy tells me that his instinct is to hit the other kid, but he knows that is not right and that he will probably get in trouble. Someone else suggested that he not actually hit, but use his size to threaten. We joked about having younger siblings go over, including a 5 year old, and tell the kid to lay off. In the end, we helped him to identify a friend who could come along with the group, so at least the dynamic will shift if he feels like he has an ally.

In truth, I’m not sure of the best way to respond. I tend to back down from a fight, I usually think it is just not worth it, and my instinct here is to just get the child removed from the situation. I know that you can’t get switched out of every group, though, so I’d love to hear from others — how do you teach your child to respond to a bully? what do you tell a child who has been told that no one likes them? what do you do when the supervising adults involved are either oblivious or not able to do their jobs? Because you know in your heart that as much as you tell them it isn’t true, even as you give them proof that it isn’t true, you can’t make it unsaid, and those scars of childhood can add up into a hard, defensive heart.