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Trait: Sturdy -----> Sand Force
EVs: 252 HP / 56 Def / 200 Sp. Def
- Stealth Rock
- Heavy Slam
- Roar / Toxic / Sandstorm
- Earthquake / Rock Slide / Stone Edge
The Mega-Evolution for Steelix made it a lot more viable this gen. It got significant boosts to its Attack and Sp. Def stats. However, the most notable stat is its sky-high Defense stats. Although it faces fierce competition from Mega-Aggron, its typing gives it nice defensive options and also counters the infamous Volt-turn teams.
EVs are meant to give Steelix decent bulk. Impish Nature is required to survive certain attacks, such as Darmantian's Flare Blitz (even when boosted by Life Orb). The rest are put into HP and Sp. Def to further boost its walling capabilites.
Stealth Rock is a necessity on any M-Steelix sets.
Heavy Slam is a requirement because it gets STAB and good power. Gyro Ball is also an option, but it has less PP and is generally bad when you can have Heavy Slam (especially since Steelix is quite heavy).
Roar allows Steelix to phaze out any opposing Pokemon. It is important to note that Dragon Tail is an inferior option because of the rise of Fairy-types and its 90% accuracy. Toxic is also another option that cripples other walls, such as Vaporeon. Sandstorm is generally an inferior option to both Roar and Toxic, but it can be used to take advantage of its ability.
Earthquake gets good STAB, while Stone Edge can be used to deal with Flying-types. If you're worried about the accuracy, Rock Slide is also an option.
Steelix is able to set up SR reliably. After doing so, you should use Roar to phaze out and rack as much as damage as possible.