Steelix is an anomaly in that it's a primarily defensive Ground-type. It has incredible Defense and resists Normal-type attacks, letting it counter Snorlax, and its Steel / Ground typing gives it a crucial Electric immunity paired with neutrality to Ice-type attacks, thereby making it a safe switch-in to Electrics with Hidden Power. It's also one of the best go-to defensive phazers for responding to threats such as Perish trap Misdreavus and Baton Pass Jolteon. However, while Steelix surpasses other Ground-types defensively, it falls incredibly short of their offensive prowess with an Attack stat of 268. Still, Steelix is not totally inept. With Spikes down, it can use Roar to shuffle the opponent's Pokemon with the best of them, and it can set up Curses to pressure the opponent or even sweep in a late-game situation where all the Pokemon capable of hurting Steelix are KOed. It also gets Explosion to instantly take out a wall. Overall, Steelix is an excellent defensive pivot and emergency phazer that, while offensively handicapped compared to other Ground-types, can still threaten foes.
Unlike other Ground-types, Steelix plays more as a defensive pivot than an offensive powerhouse thanks to its combination of great defensive typing and woefully mediocre Attack. STAB Earthquake is, aside from the Electric immunity, one of the biggest perks to being a Ground-type, so it's a must on Steelix to allow it to threaten relevant damage against foes. Curse is also required, as it allows Steelix to bolster its Attack to levels at which neutral Earthquakes actually threaten significant damage. Curse is the main way Steelix manages to create offensive pressure, especially in conjunction with Roar. Roar allows Steelix to prevent itself from being phazed by Pokemon such as Skarmory while it is setting up Curses, and furthermore can be used to shuffle the opponent's team to either scout or accumulate Spikes damage. Roar is also fantastic defensively, as Steelix is capable of countering several threats such as Curse Snorlax, Perish trap Misdreavus, and Baton Pass Jolteon, Espeon, and Umbreon.
The last move depends on what you want out of Steelix. Explosion allows Steelix to break walls instead of simply applying offensive pressure. After a Curse, a full-health Steelix can survive Suicune's Surf and guarantee an OHKO in return with Explosion. Other threats, such as Zapdos and Miltank, are guaranteed to fall to a +1 Explosion, and Zapdos in particular is likely to be OHKOed and takes at least 93% damage from an unboosted Explosion. Explosion can also come in handy against Snorlax, even Fire Blast variants that Steelix otherwise struggles to beat. An unboosted Explosion does 80-94% to Snorlax, so you will need a little bit of residual damage to make this into an OHKO. Skarmory can also be KOed by Explosion if Steelix is able to set up enough Curses beforehand—at least 4 Curses are needed to guarantee an OHKO against an unboosted Skarmory, whereas 6 are needed to have a roughly 70% chance of OHKOing a +1 Skarmory. On the other hand, Rock Slide is a more conservative option that allows Steelix to 3HKO Zapdos after a Curse without needing to Explode. Furthermore, Rock Slide allows Steelix to sweep late-game when none of the opposing Pokemon can hurt it, even if the opponent has an Earthquake-immune Zapdos or Skarmory. Rock Slide can also severely punish Charizard switch-ins without the risk involved with Explosion. However, it's important to note that outside of trying to catch Charizard, Rock Slide is useless without Curses behind it, as it fails to even 3HKO Zapdos without a boost. Finally, Body Slam is an option for Steelix's last moveslot. Body Slam does not threaten to outright KO a wall the way Explosion can, and since it only does 24-28% to Skarmory when Steelix is at +6 Attack, it does not allow Steelix to sweep, either. However, Body Slam allows Steelix to spread paralysis, thereby allowing it to support its team much more effectively than it would have been able to otherwise.Team Options & Additional Comments >>>
Curse could be foregone to allow both Body Slam and Explosion on the same set to maximize Steelix's potential for immediate offensive impact. With a set of Earthquake, Body Slam, Explosion, and Roar, Steelix can still Spikes shuffle and phaze out dangerous threats while both spreading paralysis and having Explosion for walls such as Suicune or Zapdos. However, Steelix is generally better suited to building pressure by setting up Curses. Iron Tail could be used as an alternative STAB that, by virtue of the lack of Pokemon immune to it, actually allows mono-attacking Steelix to sweep at +6. However, Steel is in general a terrible attacking type, and furthermore it is only 75% accurate and also incapable of forcing Raikou to switch out. Substitute can be paired with Curse and Roar to dodge Growl and Charm while setting up a sweep. Iron Tail is the ideal attacking move on such a set to ensure nothing can wall a +6 Steelix. Bind can be paired with Explosion to make sure Steelix takes out the appropriate target.
Checks and Counters
Water-types are the chief counters that force Steelix to switch out while taking little damage in return from Steelix's Earthquake. Suicune, Vaporeon, Cloyster, and even Starmie are all excellent switch-ins to Steelix. Exeggutor is also a fine switch-in to Steelix that resists Earthquake while threatening to use Sleep Powder or whittle away its health with Psychic, Giga Drain, or Hidden Power Fire. Heracross, while not as common, can switch into Steelix, resists Earthquake, and can whittle away its health with Seismic Toss. Some more passive switch-ins to Steelix are Growl Miltank and Charm Umbreon, which shut Steelix down by negating the Attack boosts it gets from Curse. However, these counters cannot actually hurt Steelix, thereby allowing it to gain Leftovers recovery or switch out to a threatening special attacker, such as Zapdos, that does not mind taking a Growl or Charm on the switch.
Steelix is also countered, to an extent, by Flying-types. Zapdos and Dragonite, especially if they use Hidden Power Water or Fire Blast, respectively, are great answers to Steelix. Their Flying typing, at the very least, forces Explosion Steelix to trade lives with them, although switching one of these Pokemon into a Rock Slide Steelix is a dicey proposition, especially if Steelix uses Curse on the switch-in to enable the Rock Slide 3HKO. Charizard is an unconventional Pokemon, but it is similar to Zapdos and Dragonite in that it at worst trades with Explosion Steelix while struggling to switch into Rock Slide Steelix. Charizard, however, can OHKO Steelix with STAB Fire Blast, so it fears switching into Rock Slide more than it fears switching into a Curse. Skarmory, too, is similar in that it forces Explosion Steelix to trade with it, while Rock Slide Steelix ultimately beats Skarmory if it is the only decent answer to Steelix left. However, Steelix must hit Skarmory on the switch with Rock Slide, as it must use Roar against an active Skarmory to avoid being phazed and losing its boosts. Thus, if Skarmory is paired with another Pokemon that can easily tank Rock Slides, Skarmory can attempt to stall Steelix out of PP. This is made even easier if Skarmory is using the uncommon Sand-Attack set.
Certain physical threats can also come in to capitalize off of Steelix's weaknesses. Marowak can handily 2HKO with Earthquake, although its Earthquake is downgraded to a 3HKO if Steelix has a Curse set up, so Marowak cannot switch into an active Steelix and instead must switch in as Steelix switches in. Machamp is similar, except its unboosted Cross Chop only 3HKOes, although the high critical hit ratio means that Cross Chop can potentially OHKO Steelix. Again, though, a Steelix that has set up Curse can tank these hits (unboosted Cross Chop from Machamp only does 30-35% to Steelix after a Curse), so Machamp must switch in as Steelix switches in, rather than switching into an active Steelix. Of course, these Pokemon and several others could also be using Fire Blast to roast Steelix for at least a 3HKO on the special side, meaning that even if they are brought in by Roar against a Steelix with several Curses, they could stay in despite the Defense boosts and still do heavy damage. Even Pokemon that Steelix normally counters, such as Snorlax, can easily turn the tables on Steelix with Fire Blast.