I'm usually not too much of a fan of these alternative battle modes (horde/rotation etc) but I really enjoyed doing the Battle Royal in S/M.
Perhaps it was the setting of being in a ring like stage or the cool music (These two reasons Morri said reminded her of Glitzville in Paper Mario) but it was something I played through until I reached the top rank. It didn't seem all that difficult either. Just knowing when to switch/protect and which Pokémon to gang up on. Golispod was a big help with his priority moves.
Funnily enough, in the last rank I had to do, there was one other opponent with a Weezing. Out of my love for the bugger I left it alone but amazingly it seemed to team up with me, never once going for me and instead helping take out some of the others with Will O Wisp and attacks. A good a tank he was, he fainted in battle to which I saluted with a tear in my eye. Remember the fallen!
I've watched some videos online of 4 people playing together and it looks like loads of fun. Need to test it out at some point.