Trying out this team in OU(non-pokébank). Currently crawling my way out of elo hell at 1405, with a 43/4 W/L. Just barely started to see some non in-game teams.

Zygarde @ Leftovers
Ability: Power Construct
EVs: 252 HP / 252 SpD / 4 Spe
Adamant Nature
- Coil
- Rest
- Thousand Arrows
- Sleep Talk

Toxapex @ Black Sludge
Ability: Regenerator
EVs: 252 HP / 252 Def / 4 SpD
Bold Nature
IVs: 0 Atk
- Baneful Bunker
- Toxic Spikes
- Haze
- Scald

Tapu Bulu @ Assault Vest
Ability: Grassy Surge
EVs: 248 HP / 252 Atk / 8 SpD
Adamant Nature
- Wood Hammer
- Megahorn
- Nature's Madness
- Stone Edge

Xurkitree @ Choice Scarf
Ability: Beast Boost
EVs: 252 HP / 252 SpA / 4 SpD
Modest Nature
IVs: 0 Atk / 30 Def
- Discharge
- Dazzling Gleam
- Volt Switch
- Hidden Power [Ice]

Celesteela @ Leftovers
Ability: Beast Boost
EVs: 252 HP / 252 Def / 4 SpA
Bold Nature
IVs: 0 Atk
- Flamethrower
- Flash Cannon
- Leech Seed
- Protect

Mandibuzz @ Leftovers
Ability: Overcoat
EVs: 252 HP / 252 Atk / 4 SpD
Adamant Nature
- Roost
- Brave Bird
- Foul Play
- Knock Off

In 95% of the matchups I lead with Bulu into Zygarde or Toxapex. I'm open to replace Nature's Madness, but there aren't many better options for Bulu really. Celesteela and Mandibuzz are also doing good.
Xurkitree isn't really pulling his weight though, any suggestions for replacements would be welcome.