Greninja is good it the battle tree, but only if you carry a good Pokemon to switch into. The battle tree is surprisingly different from competitive, and if you go in with 3 frail sweepers, your going to lose (equally surprising, you can stall with Shuckle). You often need to take hits because of the sheer number of defensive sets and the game trying to counter your sets. Some rounds, you get an eeveelution team or a mono Ice, and other you get Blaziken, Garchomp, and Mega Lucario. :P
But there is no doubt that Luvdisc is far outclassed by a lot of Water types and nearly unusable in the battle tree. :P