Gyms has been nerfed now, I'd manage to beat them few time on my own. Also, please refer to this website to know all of the detail and how to beat the gym leaders. https://www.dltk-kids.com/pokemon/battle.htm
An update on this i am developing a feature that allow this concept. Using the claiming system that will soon offer "renting a claim". Once that put into place onto the server this feature will be add it.