Battle Moveset

A Good Moveset for Steelix would have to be this:

Attacks:

Toxic
Protect
Earthquake
Rock Slide

Items Attached:

Leftovers

Preferred Nature:

Adament/Impish

Strategy Using Steelix

Steelix has one of the best base stats in defense in the game. This pokemon can take alot of physical hits and its attack isn't too bad either. The item, Leftovers will prove useful on this pokemon since it will be able to take hits and Leftovers will heal some of the HP lost.
For a nature on Steelix, I would advice either Adamant or Impish. While Adamant benefits attack but hinders special attack, Impish increases Def and reduces special attack, personally, I would pick Impish.
The EV's on this pokemon is intended to increase the pokemon's ability to stay in the game. With poor special defense this pokemon wouldn't last a fire attack.
For its ability, rock head is useless here since it doesn't have a move that gives any recoil damage. Its other ability, Sturdy, is pretty useless unless your opponent has One hit Knock out moves, which should be rare.
If you think your opponent could possibly switch out for a pokemon with a fire type move or is a fire type pokemon, use toxic and if you feel you have a chance to survive a fire attack, attack with either earthquake or Rock slide. Otherwise, if you don't feel you have the chance to survive the attack, switch for another pokemon.

EV Corner:

Max out the EVs in HP & Special Defense for Steelix with any remaining in Attack

Strategy Against Steelix

Steelix's Physical Defenses are UNTOUCHABLE...they are absurdly high so your best bet here is to get it with Special Attacks and Super Effective attacks. It'll be down in no time

Contest Moveset

A good Contest Moveset for Steelix would have to be this for the Tough Contest best with Impish, Lax, Relaxed or Bold Nature:

Attacks:

Harden
Earthquake
Sandstorm
Tackle

Items Attached:

Yellow Scarf

Strategy Using Steelix

If you do the attacks in this following order you should have very few problems; 1st - Harden, 2nd - Earthquake, 3rd - Sandstorm, 4th - Tackle, 5th - Earthquake

Locations in Games

Ruby/Sapphire Trade From FR/LG Colosseum Trade From FR/LG Fire Red/Leaf Green Evolve Onix

Animé Appearences

Steelix has had several Animé Appearences. First, Jasmine has one and used it in her battle against Ash. After that a Team Rocket Member used it in order to stop Yoshi, Dani & Jackson from getting to Raikou and saving it. After that Harrison used it in his battle against Ash and after that, A Wild one was attacking a Wild Torkoal Episode 226: Nerves Of Steelix! Special 3: Raikou! Legend Of Thunder! Episode 257: Why? Wynaut! Episode 273: Playing With Fire! Episode 274: Johto Photo Finish! Episode 334: Breaking Through Steel Valley! Torkoal VS Steelix!