How do I fight him, tell something about it?

[Khalique]

I m 14, and in 8 grade, and skinny. So there are some bullies.....not some...team of bullies which keeps bullying me, but not all of them only one fat fat guy. the leader of the team is kindda freind of me, but that fat *** keeps snatching my hairs, slapping me, and calls me with bad words, and i want to fight him, i said the leader that i will fight him in august, means i have lot of time, i also started body building and i also gained weight but that maste**** problem made me powerless but muscles aren't only thing i need strength, I fighted that fat guy many times and i noticed that not to go near him, and he can't run, and my punches and kicks have no affect on him. So now I need a some thing about how to make muscles + strength. and don't give amrtial arts thing, cuz no sensei was able to teach me. And don't give answers like don't fight or tell parents or administration, cuz this is not your country where they can help, and don't even say that don't do bodybuilding cuz all those bullies are bodybuilders, even the leader uses steroids, but i don't want to use steroids, i want natural strength and muscles, I want to fight that fat ***.