that in terms of male human and female Pokémon battling, Vaporeon is the most compatible Pokémon for fucking you over at the Battle Tree? Not only are they in the bulky as fuck group, which is mostly comprised of leftovers and substitute spam, Vaporeon on average use Yawn and Muddy Water. this means they're large enough to be able to handle Mega Metagross, and with their impressive Base stats for HP and access to Aqua Ring, you can be rough with one. Due to their mostly water based biology, there's no doubt in my mind that a Muddy Water spamming Vaporeon would be incredibly obnoxious, so obnoxious that you could easily battle with one for hours without hitting it once. They can also learn the moves Attract, Baby-Doll eyes, Captivate, Charm and Tail Whip but they all suck shit, so it'd be incredibly easy for one to get you into the next round. With their abilities Water Absorb and Hydration, they can easily recover from fatigue with enough water. No other Pokémon comes close with this level of compatibility. Also, fun fact, if you do beat it, the game will just find another way to fuck you. Vaporeon is literally built for the Battle Tree. Ungodly defense stat + high HP pool + Muddy Water accuracy drops means it can take Mega Metagross all day, all shapes and sizes and still come for more.
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https://www.youtube.com/watch?v=aM9lmanAx1I