The pit.

I have genuinely no clue on how this pit thing works. Ive been doing it for the last month or so and the thing i dont understand about it is, how can someone thats a lower level, with less troops, beat me and take my spot and steal? (I usually use fighters and shooters with heros that buff them). Whats the best troop formation for the pit? I hear riders are really good but im still pretty new to this and would love to have some feedback!