Diglett @ Groundium Z / Life Orb
Ability: Arena Trap  
Level: 5  
EVs: 36 HP / 236 Atk / 236 Spe  
Jolly Nature  
- Precipice Blades  
- Rock Slide  
- Sucker Punch  
- Substitute / Memento
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Optional Move(s):
- Beat Up
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Diglett @ Choice Scarf
Level: 5
Ability: Arena Trap
EVs: 36 HP / 236 Atk / 236 Spe
Jolly Nature
- Precipice Blades  
- Rock Slide
- Final Gambit / Memento  
- Beat Up / Pursuit
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Diglett @ Focus Sash
Level: 5
Ability: Arena Trap
EVs: 36 HP / 236 Atk / 236 Spe
Jolly Nature
- Precipice Blades
- Sucker Punch / Memento
- Rock Slide
- Memento / Reversal / Stealth Rock  
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Overview:
Diglett's strong niche lies in its Arena Trap ability, which is the sole trapper available. It possesses a great speed, which ties with other two pokemon as the fastest unboosted pokemon. However, Diglett has mediocre Attack stat, which leave it reliant on chip damage and Z-Move to score KOs. Its lackluster bulk makes it hard to switch into anything it attempts to trap and is vulnerable to priority users.