Located at the end of Poni Gauntlet, the Battle Tree is the endpoint of your Poni Island adventures. Immediately as you enter, if you start walking, you'll be stopped by a pair of Champions like yourself: Red and Blue from, well, Pokémon Red and Blue. In order to proceed, you must battle and defeat one of them, of your choosing.
It doesn't matter which Champion you battle--and both are similar in strength. If you want, you could even fight both! To do so, simply save your game beforehand, pick one of the trainers to battle, reload your game afterwards and then pick the other trainer. Regardless of your choice, get ready for the fight of your life!
Pokémon Trainer Red
Red's team is basically a team you'd expect from somebody playing Pokémon Yellow.
First up is a Pikachu boosted by a Light Ball. This Electric rodent has high offensive and Speed stats, but paper-thin defenses. It can use Volt Tackle for STAB, Quick Attack to go first, Nuzzle to paralyze, and Light Screen to raise its party's Special Defense. Ground-types will wipe the smile from its face.
Lapras has impressive bulk, but below average Speed. Red's Lapras can use Surf and Blizzard for STAB, Ice Shard to go first, and Psychic because he ran out of ideas? Electric or Fighting-types are recommended, but you should expect Lapras to survive at least one attack. Good thing it doesn't have Sheer Cold!
The lazy Snorlax has incredible special bulk and formidable Attack, but is super slow. This one can use Body Slam for STAB, High Horsepower to hurt Rock and Steel-types, Crunch to hurt Ghosts, and Heavy Slam to squash lightweight foes. Thankfully it lacks Rest. Fighting-types are highly recommended.
Venusaur is a fully evolved Grass starter with solid special-based stats and otherwise balanced stats. The once you're facing has Leaf Storm and Sludge Bomb for STAB--the former weakening Venusaur's damage output. It can also use Earthquake to hurt Fire-types, and Leech Seed to slowly drain HP. Hit it with Flying, Ice or Psychic.
Charizard is a fully evolved Fire starter with respectable Special Attack and Speed. It can use Fire Blast and Air Slash to deal STAB damage, Focus Blast to handle Rock-types, and Will-O-Wisp to cripple physical attackers. Water and Electric-types are recommended, especially if they're special attackers.
Finally, Blastoise is a fully evolved Water starter with solid defenses. This one has Hydro Pump for STAB, Ice Beam to damage Grass-types, Dark Pulse to try and flinch, and Flash Cannon to surprise Fairies maybe? Electric-types are ideal, or Grass-types if you think they can withstand a non-STAB Ice Beam or two.
Pokémon Trainer Blue
Blue's team is essentially his final team in Pokémon Red and Blue if he didn't have a starter Pokémon.
First to enter the arena is Alakazam, an extremely agile special attacker that is terrible at taking physical hits. Blue's Alakazam can use Psychic for STAB, Shadow Ball to counter opposing Psychics, Future Sight to attack later on, and Reflect to up its party's Defense. Dark-types are ideal; Bugs and Ghosts also work well.
Machamp, the Superpower Pokémon, has incredible Attack, but low Speed. It can use Cross Chop to deal STAB damage, Poison Jab to punish Fairies, Knock Off to hurt Psychics and Ghosts (and remove your held item), and Bulk Up to buff itself. Flying-types are advised, unless you can one-shot it or tank a hit.
Aerodactyl is a rare fossil Pokémon with stunning Speed and pretty good Attack. It can use Rock Slide and Sky Drop for STAB, Crunch to pester Psychics and Ghosts, and Iron Head to annoy Fairies etc. Water and Electric-types are recommended, but if they're slow, be mindful of Aerodactyl outspeeding them.
The Kantonian Exeggutor has great Special Attack, but moves as fast as a palm tree would. This one can use Leaf Storm and Psychic for STAB--the former reducing Exeggutor's own damage. It can also use Sludge Bomb to hurt Grass-types, and Hypnosis. Bug-types are its bane, and it's weak to Fire, Ice, Flying, Dark and Ghost.
The majestic Arcanine has reasonably well-balanced stats, with Attack being its highest by a small margin. It can use Flare Blitz for damaging STAB (also damaging itself), Thunder Fang to hurt Water-types, Bulldoze to hurt Rock-types, and Extreme Speed to go first. Ground-types are the safest choice.
Finally, Gyarados boasts high Attack and respectable special bulk. Blue's Gyarados can use Aqua Tail for STAB, Ice Fang to hurt Grass-types, Crunch to eat into Psychic and Ghost-types, and Dragon Dance to boost its Attack and Speed. If you have Electric-types, use them. Either way, focus on damaging it before it dances too much.
Succeed in defeating one of the Kantonian Champions and they'll leave on their merry way. From here, you can head directly north to make use of the Battle Tree's facilities. Standing in the middle is of course the receptionist for the Battle Tree itself. If you need help, we have a section dedicated to the Battle Tree further ahead.
Anyway, for those who are new, the Battle Tree is a place where you can fight multiple trainers in a row. You don't earn experience, but instead Battle Points (BP), used to buy from BP Shops here and elsewhere. At first, you can fight without restrictions, but eventually all Pokémon will be set to Level 50, and certain overpowered Legendaries will be banned.
A Reason to Fight
The first time you reach a winning streak of 20 battles in Single or Multi Battles, you will battle Red (alongside Blue in Multi Battles). If you can successfully defeat him, he'll give you a set of Kantonian Mega Stones: Venusaurite , Blastoisite , Charizardite X and Charizardite Y . This is the only way to get them shy of trading.
Towards the west are a bunch of BP shops--some of which are very important. Most notably, the easternmost BP shop sells every other Mega Stone and includes previously event-only Mega Stones in Sun and Moon. The missing ones are the Kanto starters', which we mentioned above, and Diancite from Olivia's shop.
Also of importance is the westernmost BP shop, which is Poni Island's Move Tutor. If you were disappointed that it was absent from Poni Beach, they were saving it for until you completed the game! Here, you can learn many useful moves for competitive battling (or just to make your Pokémon stronger).
Last but not least, there's a male Ace Trainer towards the east of the Battle Tree receptionist who may not be interested in you at first. If you've hatched at least 20 eggs (gotten by breeding Pokémon at the Paniola Ranch Nursery), he will activate the "Judge" function for your PC , allowing you to identify your Pokémon's IVs.
That's all the new places that you can explore, but the fun doesn't have to end here. If you haven't already done so, you can return to some old locations to pick up some goodies now that you're the Champion. Plus you can devote your time to getting a high win streak in the Battle Tree, completing your Pokédex, etc.