Im not to sure what the context is here. Maybe you are a parent with a kid around 13 years old who's being bullied. Maybe your a kid being bullied. Maybe you're just writing a story about a kid who's 13 years old and is being bullied. I can tell you this, there is a difference between what I would recommend in reality and if you were writing a story. For now I'll just stick to the assumption that there is a real 13 year old kid with an issue with the "biggest bully in the school".
The problem with middle school and kids of the appropriate age, is that middle school can be a very daunting thing for most kids. Around that age is when a child's mind really begins developing. They start maturing, and begin going through puberty. And unless the parent has a very close connection with their child, this is also around the time the children will stop confiding things in their parents as they think the whole world is out to get them. Because of this, it can be difficult to help a child in middle school work out such difficulties, as in most cases the parents are oblivious to these things going on. As much as it may be a cop out, it's really a case-by-case basis on how to help a kid go through bullying at this age.
Now as for the kid.
I stand firm that surrounding yourself (as the kid) with friends is the best option to take away power from the bully. You comment that the kid's "friends" may keep away from the victim in fear of the bully, well if that were the case then they really aren't his friends after all, are they? A kid may make "playmates" in elementary school, but it's in middle school that kids develop actual friends. And these are typically the same friends that they continue to develop relationships/friendships with in high school. You underestimate just how loyal friends can be in middle school, as in middle school children are still pure of heart and are still innocent minded (even if it's just for a few more years). If a group of children knowingly stay away from one child because they don't want to get in the way of the bully, then they are definitely not friends of the victim. True friends would band together, regardless of if they stand up to the bully for the victim, or if they likewise become victims themselves.
As for your idea that the victim befriend another "big individual", in middle school this may not be the best idea as that would only fuel the bully's sense of power over the victim. The bully would see the victim is hiding behind a larger person, and while they may stay away from the victim when he/she is around the new "friend", they would continue to harass him/her whenever they aren't with their new bodyguard. Also, this is usually just a poor way to make a new "friend" unless there is a genuine connection between the two.