The Battle Tree has some rules that you're going to have to abide by. For those used to playing the main game, they might come as a bit of a shock, so this section will give you a thorough explanation of what to expect so that you aren't caught off guard!
The Battle Tree offers Single, Double , and Multi battles. No more Triple or Rotation. Single and Double are obvious, but Multi means you will have an AI partner or a real life friend bringing Pokemon and so it will be two trainers versus two trainers.
Additionally, you can select any of 16 tracks of music to play during the battles, making it as least-boring as possible.
Lv 50 Cap
All battles will have all Pokemon scaled down to Level 50 . This, naturally, evens the playing field by making sure everyone is at the same level, and is also the biggest reason you absolutely will have to train your Pokemon properly and prepare a solid strategy beforehand.
In addition to this, you and your opponent will have the same number of Pokemon, so you no longer have a numerical advantage.
Single battles are three vs three, doubles and multi are four vs four. In doubles you will bring four Pokemon, but in Multi you only bring two and your AI partner will contribute another two (or your real life friend, if you've got one willing to join you).
No duplicates...of anything
Not being able to bring two of the same species of Pokemon is probably a no-brainer, but you also cannot equip two of the same item to two Pokemon either. Leftovers teams aren't going to be a thing, sadly. This means you'll have to be more creative with your item sets to ensure that you can enter.
Note that duplicate Pokemon means they have the same Pokedex number. So bringing multiple forms of Oricorio, for instance, isn't allowed (why would you do that anyways, though?). Same goes for Lycanroc's two evolutions, and so on.
As an addendum, you also cannot bring certain Legendary Pokemon or "Mythical" Pokemon. The list is as follows and includes National Dex Pokemon:
Banned Legendary Pokemon
Mewtwo, Lugia, Ho-Oh, Kyogre, Groudon, Rayquaza, Dialga, Palkia, Giratina, Phione, Reshiram, Zekrom, Kyurem (all formes), Xerneas, Yveltal, Zygarde (all formes), Cosmog, Cosmoem, Solgaleo, Lunala, Necrozma.
Banned Mythical Pokemon
Mew, Celebi, Jirachi, Deoxys, Manaphy, Darkrai, Shaymin, Arceus, Victini, Keldeo, Meloetta, Genesect, Diancie, Hoopa, Volcanion, Magearna, Marshadow.
Note that not all legendary Pokemon are banned. Here are the ones that you can bring into the Battle Tree:
Allowed Legendary Pokemon
Articuno, Zapdos, Moltres, Suicune, Raikou, Entei, Latios, Latias, Regirock, Regice, Registeel, Uxie, Mesprit, Azelf, Tornadus, Thundurus, Landorus, Cobalion, Virizion, Terrakion, Tapu Koku, Tapu Lele, Tapu Bulu, Tapu Fini.
All Ultra Beasts are allowed: Nihilego, Buzzwole, Pherosma, Xurkitree, Kartana, Celesteela, and Guzzlord.
Streaks are everything
The most important thing about the Battle Tree is your win streak . This means you have to win consecutive matches without a single loss. Just one loss will put you back at 0 streak!
When you first enter the Battle Tree, you will have to fight the standard Single and standard Double/Multi, which maxes out at 20 battles. After that, you will unlock the Super versions, and that's where you will find the true difficulty.
Similar to the Battle Maison in X, Y, Omega Ruby and Alpha Sapphire, you can save and stop anytime instead of having to go through seven battles before saving.
Every 10 matches at the Super level, you will fight a Battle Legend . These are characters that debuted in this game, or nostalgia characters from previous games. If you successfully defeat a Battle Legend, you can scout for them in Multi Battles at the cost of 10 Battle Points (BP).
The next section details the Battle Legends you can face.