Moves
Earthquake is a reliable STAB move that allows Diglett to take out the Electric-, Steel-, and Rock-types that it aims to trap. If Groundium Z is run, Diglett can use Tectonic Rage, a powered-up version of Earthquake that will take out Pokemon that Earthquake could not normally KO, such as fast Mienfoo variants and slightly weakened Foongus and Staryu. Rock Slide allows Diglett to take out weakened Pokemon that are immune to Earthquake, such as Vullaby. Sucker Punch is useful against weakened or frail Choice Scarf users, such as Chinchou and slower priority users, such as Doduo. Sludge Bomb is an alternative that lets Diglett trap Cottonee and deal decent damage to other Grass-type Pokemon that aim to revenge kill it. Substitute allows Diglett to play mind games with Sucker Punch users such as Pawniard and, potentially, gives Diglett free turns against foes Choice-locked into Electric-type moves. Memento lets Diglett give a partner a free setup opportunity against the foe of its choice, with the opponent unable to switch out. Sludge Bomb remains an option in this slot, giving Diglett the ability to trap the Pokemon mentioned above. Stealth Rock can be used as well on teams that need it, as Diglett can use it after trapping and taking out a Pokemon.
Set Details
The given EV spread maximizes Diglett's Attack and Speed, with the remaining EVs giving Diglett an extra point in HP. With maximum Speed investment and a Jolly nature, Diglett hits 20 Speed, outspeeding or tying with the entire unboosted metagame. Groundium Z is the preferred item to trap and remove key targets such as Knocked Off Mienfoo and Foongus from a much higher HP than otherwise possible. Life Orb is a viable alternative if the team already has a Z move, as it grants Diglett more consistent damage output. Arena Trap allows Diglett to be the metagame's top revenge killer, as it keeps grounded targets from switching out, allowing Diglett to KO them.
Usage Tips
Diglett should be used to trap and revenge kill specific Pokemon throughout a match. It should almost never be switched into attacks, due to its frailty, but instead brought in by a slow pivot or after the opponent KOes a Pokemon. Scout out opposing Pokemon that may be equipped with a Choice Scarf early on, as Pokemon such as Chinchou and Magnemite that Diglett can often trap can also surprise and outspeed it with a Choice Scarf. If a foe is equipped with a Choice Scarf, Diglett should wait until it is within range of a Sucker Punch KO before trapping it. Be aware that, even with a Life Orb, Diglett is not particularly powerful and with a Groundium Z is even less so when not using Tectonic Rage. Thus, be careful of opposing Pokemon that could come in after Diglett KOes a Pokemon and use it as setup bait, including Timburr, Snivy, and Pumpkaboo-S. If Groundium Z is run, it should be used to power up Earthquake once, when Diglett needs to provide a KO for its team that Diglett's other coverage moves cannot provide. After getting the KOs it needs, Diglett can use its high speed to get off a Memento or Stealth Rock before being KOed.
Team Options
Pokemon that appreciate Diglett's ability to remove Rock-, Steel-, Fire-, and Electric-type foes make the most obvious partners. Flying-types such as Vullaby and Rufflet appreciate Diglett dealing with all of these, as do Normal-types such as Aipom. Fairy-types such as Spritzee, which appreciate Diglett's ability to remove Poison- and Steel-types, make good partners, as do Grass-types, such as Snivy and Pumpkaboo-S, which appreciate the removal of Fire-, Steel-, and Poison-types. Slow pivots that can bring Diglett in safely make good partners as well. Mienfoo is one such option that also appreciates Diglett's ability to remove Ponyta and Poison-types, significantly damage most Fairy-types, and KO Cottonee if running Sludge Bomb. Chinchou also pairs fairly well with Diglett, as Diglett can trap opposing Chinchou for it to pivot more easily. If Diglett is running Groundium Z, Pokemon that appreciate its ability to KO bulky Fighting-types with Tectonic Rage make good partners, especially Carvanha and Scraggy. Pokemon that can take on the bulky Flying-types that don't mind Rock Slide are good partners for Diglett, especially Electric-types such as Elekid and Magnemite. Entry hazard support from the likes of Onix or Ferroseed helps Diglett safely secure more KOs and can break Magnemite's Sturdy to allow Diglett to trap it more safely. Similarly, Knock Off users, including Mienfoo and Pawniard, help Diglett secure more KOs it couldn't get otherwise by removing Eviolites and Berry Juices from foes, and can also remove Choice Scarves to help Diglett trap Pokemon such as Magnemite and Chinchou. Sticky Web setters, specifically Surskit, help Diglett by allowing it to outspeed grounded Choice Scarf Pokemon and trap them reliably.
