Mega-Steelix - Curse Set / Hazard Set Up
Steelix - Item(Steelixite) - Ability(Sand Force) - Nature(Sassy, -Speed +Sp.Defense)
EV Spread - 252 HP, 252 Sp.Def, 6 Def/Atk
Moves - 1. Curse - This boosts Mega Steelix's already amazing defense, and a nice attack raise.
2. Stealth Rock / Sandstorm/ Toxic - If you already have a Hazard Setter, then Sandstorm is a great way to boost Steelix's moves' damage while gradually hurting the enemy if it isn't resistant. STAB paired with 1.3x the power can pack huge punches on sweepers, espesially with a Curse boost. If you would prefer more of a wall, then Toxic is a better choice to do it.
3. Gyro Ball / Heavy Slam - Gyro Ball paired with Curse is amazing. A raise to attack and a lower to speed doubles the power of this move, and with Sandstorm up as well, it will do loads of damage. Still, if you are a bit iffy on speed and prefer weight and PP, you can always go for Heavy Slam.
4. Earthquake - Great coverage and damage. STAB and Sandstorm makes this move deadly to be hit by.
Mega-Steelix offers great defensive and offensive capabilities, as well as hazardous ones. Its' ability could be better, I think we can all agree on that, but it is still decent. Curse can turn it into a dangerous and hard-to-kill force that can not only easily take physical hits, but can start to land hard ones as well. With a lack of recovery however, it can still gradually be killed. To help this, you could always go for a Rest and Sleep Talk, but that is another set. While it is up and running, it can pack a serious punch, and with good investments into defenses, he will be up a lot longer.