Sandslash

Type Tier
Ground UU
Statistics
Stat
HP
75
353
Atk
100
298
Def
110
318
SpA
45
188
SpD
55
208
Spe
65
228
Name Item

SD Sandslash

Leftovers
Moveset
~ Swords Dance
~ Earthquake
~ Rock Slide
~ Substitute

Swords Dance obviously gives Sandslash some more attacking power and the Earthquake / Rock Slide combo is a given. Substitute provides some more setup time for Sandslash. Its Speed is high enough to get a Substitute up before Exeggutor can Sleep Powder or Giga Drain you, for example. This set works best with plenty of paralysis support from the rest of your team.

Other Options

Body Slam is a great option over Substitute since it provides the added paralysis that Sandslash usually needs to sweep teams and beat Water-types like Starmie. Sand-Attack is an option for annoyance purposes against something like Skarmory who will attempt to Whirlwind you. Rest for RestBell teams. Counter is a cool move since it works on Hidden Power so something like Hidden Power Zapdos can get Countered for the KO since Sandslash has enough Special Defense to take the blow and still be usable. Sandslash could use Hidden Power Bug for Grass-types, but you'll have to hit them on the switch.

Opinion

Sandslash is a vastly underrated Ground-type in GSC and mainly because Marowak is more powerful. However, Marowak also gets KOed easier and doesn't match Sandslash's Speed. Sandslash is the fastest pure Ground-type after Dugtrio, but Sandslash can actually do damage and set itself up. Once its Attack is boosted, Sandslash can do some decent damage to many of the slow, tankish Pokémon that are often played in GSC. With paralysis support, this thing does usually pretty well.

Counters

Most Water-types, such as Quagsire, have good Defense with which to take Earthquake, and while Sandslash's Defense is great, its Special Defense is not. A strong Surf will knock it out quickly. Grass-types like Meganium and Tangela are resistant to Earthquake and also reply with super effective attacks. Note that if Sandslash is behind a Substitute, they cannot break it with Giga Drain because Giga Drain fails on Substitute.