I actually EV-trained it under the mistaken assumption that its base Speed was really low, so I just maxed its Attack and split the rest evenly between Defense and Special Defense. Still manages to outspeed a fair amount of things on my end. That said, I should note that the Battle Tree has pretty consistently screwed me over once I get past 20 battles, so it's hardly unstoppable. And against actual human players I reckon it'd be pretty much useless. But it has swept quite a few teams in the Tree and is easily the most effective means of farming BP I've found so far. Another thing I should point out is that the CPU tends to be really dumb at times and try to use stuff like Calm Mind on the first turn or two, thus letting me set up multiple Swords Dances without any danger. And I get hit with Swagger a lot too, to which I basically just go "Thanks for the free boost" and counter with a Shadow Sneak.
Ah, thanks for the explanation. That makes sense. I think after spending all that time in the Battle Tree trying to reach 50, I automatically judge teams / setups on whether they're actually viable to survive 50 consecutive RNG hax battles. It certainly helps for BP farming to make it to the 20s often, but why not try a team that could potentially could go further? Like I have only retried a few times since breaking 50 but the team I used would most often lose in the 40s, sometimes in the 30s but not too often, and almost never in the 20s or below (if anything it would be to very bad luck on the guest appearances in battle 20 or 30). It definitely feels good once you're making 5+ BP per win.
I would also like to point out that it's not just about toughing it out long enough to survive bad RNG. I mean for one there's the trial-and-error experience of knowing how to handle various situations and opponents. But the battles actually do get noticeably harder as you progress. It becomes pretty apparent over time, in several ways. The most obvious is probably that mega evolutions start happening way more often. I'm not sure they happen at all before 30, other than the guest battles and even then not always. A lot of those can be super hard to consistently deal with. I think I mentioned some stuff earlier like Mega Lucario, Mega Charizard Y, Mega Gallade, and Mega Lopunny… those are just a few that gave me trouble at different points, but there are plenty more that may be a pain given your setup. Especially after 40 there's no shortage of crap like Mega Tyranitar, Mega Garchomp, Mega Kangaskhan, Mega Gengar, Mega Swampert, Mega Pinsir, Mega Aerodactyl, Mega Blaziken, Mega Abomasnow, Mega Absol, you name it.
But also, and probably more importantly, the enemy movesets as well as the AI behavior improve significantly. All those dumb power-ups people do to let you Swords Dance mostly stop in favor of status moves or hard damage, and in general things I usually dispatch easily early on in the ladder start using moves or strategies that legitimately threaten me. Sometimes it's surprisingly annoying stall mons, with enough foresight to have been set up with a Baton Pass even. Sometimes it's someone stealing my Leftovers and replacing it with a Flame Orb. Mostly what gets me is when they unleash constant brutal moves that usually have a downside like a massive attack drop but that carry enough momentum to cripple. Stuff like Overheat. Whereas my Xurkitree usually easily KOs Arcanine early on because it uses Sunny Day or some other mediocre attack, later on it will just mercilessly spam Overheat. Mega Lucario will spam Close Combat (seriously fuck Mega Lucario, that's one thing Mimikyu can counter thank god).
Well that was quite some rambling but yeah I guess my point is don't assume a team that can make it to 20 wins can necessarily make it to 40 or 50 wins.