As far as I can tell, this is a Hackamons Battle. If you look closely, the Pokemon are level 9999, which is impossible without hacking. He also has a Shiny Buzzwole that knows Sheer Cold and a Greninja which knows explosion, which are unobtainable. I also believe that a custom battle is where the host makes the rules. That explains the level 9999 Pokemon, illegal moves, and the shiny Buzzwole.