Charizard isn't that great in Battle Tree, mostly because it's slow, can't set up, and loses too easily to several rock opponents.
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A similar Pokemon that fixes these issues is Blaziken. Here's a set I found on the Internet.
Blaziken @ Focus Sash
Ability: Speed Boost
EVs: 252 Atk / 4 SpD / 252 Spe
Adamant Nature
- Blaze Kick
- High Jump Kick
- Hone Claws
- Stone edge
A good teammate for this Blaziken is Aegislash. It can switch into a lot of stuff, including most flying and psychic attacks that threaten Blaziken.
Aegislash @ Weakness Policy
Ability: Stance Change
EVs: 252 HP / 252 SpA / 4 SpD
Quiet Nature
- Shadow Sneak
- King's Shield
- Shadow Ball
- Flash Cannon
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