Moves
Curse is a great boosting move that increases Steelix's Attack and Defense and lowers its Speed. All three of these effects benefit Steelix: raising Attack directly increases the power of its moves, lowering Speed increases the power of Gyro Ball, and boosting Defense buys it more time to Curse again before it takes critical damage. Gyro Ball is the main STAB attack to be used. Because Steelix's Speed is lowered by Curse, Gyro Ball is usually at maximum power. After one Curse, it is able to OHKO Mega Kangaskhan. Earthquake is a very powerful secondary STAB move and should be used primarily to damage Pokemon that resist Steel. Rock Slide and Stone Edge are options to nab Pokemon such as Talonflame and Thundurus that do not care about Gyro Ball or Earthquake. Stone Edge is more powerful, but it has a significantly greater chance to miss.
Set Details
You should want Steelix to be as slow as possible in order for it to fully benefit from Trick Room and Gyro Ball, so no Speed EVs and 0 Speed IVs are used. Instead, those EVs can be reallocated into HP to make Steelix as bulky as possible. 252 Attack EVs make Steelix as powerful as possible. Steelixite allows Steelix to Mega Evolve to Mega Steelix, granting a significant boost to its Attack and Defense. Sturdy and Sheer Force are the two options for abilities before Mega Evolution, as Steelix uses no recoil moves for Rock Head. While Sheer Force boosts the power of Rock Slide, Sturdy is generally the better choice, as it can be useful in a last Pokemon situation. For example, against a boosted Volcarona or Charizard, Steelix can take a hit and KO back. Most of the time, though, you will want to Mega Evolve Steelix straight away. Upon Mega Evolving, Steelix gains Sand Force. However, it is not something that can really be taken advantage of, as Tyranitar and Hippowdon are poor teammates that stack weaknesses. Finally, a Brave nature is chosen to increase Steelix's Attack while lowering its Speed to increase the power of Gyro Ball.
Usage Tips
This Steelix set is more of a sweeper, as it needs the boost from Curse to start doing serious damage. However, in some matchups it can attack right away and be effective, such as against certain physically-based leads like Kangaskhan, Garchomp, and Mamoswine. Due to its low Speed, you should expect Steelix to take a hit before it can use an attack. It can take most physical hits comfortably, but it will get worn down over time, especially if it gets burned. However, if Cresselia has Lunar Dance, you can afford to let Steelix get worn down and then revive it and attempt a sweep in the late-game. Having Cresselia use Lunar Dance immediately after Trick Room to bring Steelix in is usually the best option since you will want to use Steelix under Trick Room, and if it seems Steelix is able to sweep, it ensures Steelix has enough health to set up. Curse is safer than it might seem at first; since it boosts Defense as well, it can give you more time to set up on physical attackers. Against a Mega Kangaskhan or Mega Salamence, do not be afraid to take one turn to get hit and then set up Curse, even if doing this uses a turn of Trick Room; one turn is usually all you need. However, Steelix cannot use Curse carelessly; only use it when you know it can pull off a sweep. If you are forced to switch Steelix out, it will significantly hurt your momentum. Nonetheless, once Mega Steelix has a Curse boost and is under Trick Room, it will be able to OHKO most Pokemon and pull off a sweep.
Team Options
Cresselia is an absolutely mandatory partner. It is a general defensive threat that has good synergy with Steelix thanks to its high defenses, good typing, and Levitate, making it able to come in on the Fighting-, Ground-, and even Water-type attacks that hit Steelix super effectively. It also sets up Trick Room for Steelix and gives it a free switch and healing with Lunar Dance, which is essential, as Steelix is easy to wear down over time. Both Technician and Poison Heal variants of Breloom are useful helpers to Steelix. Poison Heal Breloom can act as a status absorber in order to take random burns or sleep-inducing moves thrown at Steelix. Furthermore, Breloom can help take down Hippowdon and Suicune, which otherwise have a good matchup against Steelix. Technician Breloom trades the ability to absorb status in favor of the ability to exert offensive pressure. Serperior and Amoonguss are other Grass-types that have good type synergy with Steelix and can take down Hippowdon and Water-types. Amoonguss also enjoys having Trick Room around due to its low Speed. Lapras is a Water-type that is able to counter Suicune and Garchomp. Porygon2 is a good back-up for Cresselia, making the three-Pokemon team quite bulky. With Download, Porygon2 can also deal with Pokemon such as Rotom-W, Rotom-H, and Garchomp while also being able to set up Trick Room for Steelix. Sylveon's weaknesses are covered by Steelix, and in return it has high Special Defense to take hits aimed at Steelix. It can use powerful Hyper Voices to wear down the opposing team as well and can even benefit from Trick Room. It also has a lesser-used option, Safeguard, to protect Steelix from status as it sets up for a sweep. Mega Charizard X is a good idea for a backup Mega to replace Steelix in certain cases, as it has great synergy with the rest of Steelix's team options, such as Cresselia and Breloom. In Team Preview, Steelix tends to lure out opposing Water-types, which Mega Charizard X can usually capitalize on. It is also a good Mega to use if it seems that it will be difficult to set up Trick Room against the opposing team. Talonflame and Hydreigon are similar Pokemon that can be useful outside of Trick Room to act as stallbreakers with Taunt. Hydreigon can take care of Rotom-W and Hippowdon, while Talonflame can still be useful inside of Trick Room thanks to Gale Wings.