Level 100 Statistics (see level 5, 50, 100)
Steelix makes for an excellent wall as he can sponge attacks from a large number of standards, can beat Metagross and most Tyranitar with STAB Earthquake, and can use the powerful Explosion to take out a threat. He can also phaze threats such as Snorlax and Baton Pass Zapdos while beating any Raikou that lacks Hidden Power Water; in other words, virtually all of them. However, he isn't used much because of his subpar offenses; base 85 Attack is not impressive at all, and vice versa, quite a few standards take little damage from its attacks. He is far from unusable, though; he is one of the best BL Pokemon available.
Steelix can easily switch into Normal- and Flying-type attacks, most Electric-types, Metagross, and Snorlax, and then attack accordingly. Earthquake is Steelix's most reliable STAB move, while Toxic cripples Water-types and other bulky Pokemon he cannot otherwise hurt. Hidden Power Rock hits Flying-types, which are immune to Earthquake, but Toxic is generally the preferred option in the second moveslot. With cleric support, Steelix can gain a new lease of life with Rest; alternatively, Explosion allows him to go out with a bang when his job is done. However, the greatest flaw with Steelix as a wall is his lack of instant attacking power.
A Choice Band set somewhat patches up Steelix's low Attack, but he's still slow and unable to hurt common Water-type switch-ins; he also greatly misses Leftovers recovery. Still, Steelix is easy enough to switch in and can punch holes in the opponent's team with some prediction. Earthquake is Steelix's most powerful STAB attack. Iron Tail and Hidden Power Steel are both options for his Steel-type STAB, and the choice between them comes down to a simple power vs. accuracy argument; the former is preferred as it is Steelix's most powerful option against Gengar, Aerodactyl, and Regice. There are three options for the third moveslot: Double Edge is a powerful move with good neutral coverage, while Hidden Power Rock and Rock Slide deal heavy damage to Zapdos. Finally, Explosion provides one last powerful hit when Steelix has been weakened.
Block can be useful for trapping and severely weakening a Water-type switch-in. Steelix can usually survive the Surf to use Explosion, which opens up a sweep for an attacker that is walled by bulky Water-types, such as Salamence. Psych Up can let Steelix steal Curse boosts from Snorlax or even Regirock; he can then Roar them out or just beat them one-on-one.
Checks and Counters
Skarmory, Cloyster, and Forretress can set up Spikes in Steelix's face easily, though Cloyster must watch out for Toxic. In general, any Water-type with decent defenses can come in easily, shrug off Steelix's hits, and OHKO or 2HKO it with Surf; examples include Suicune, Swampert, Milotic, Slowbro, and Vaporeon. Moltres can OHKO Steelix with any Fire-type move as long as it avoids Rock Slide and Hidden Power Rock.
Fire- and Water-type attacks hurt Steelix a lot, unboosted or otherwise, and any decent neutral special attack will also seriously him. His physical weaknesses aren't quite as big of an issue, but a boosted Brick Break or Earthquake will deal heavy damage; Choice Band sweepers such as Aerodactyl, Medicham, and Heracross thus make good checks.