In ORAS, Mega Steelix came out and the first thing you might have noticed is that its overall bulk is better than that of Mega Aggron. Sounds great, right? We all know how well Mega Aggron can take a hit. But unfortunately, Mega Steelix ran into a few problems. First of all, it has Sand Force which, while nice in a Sandstorm, isn't as consistently useful to a physical wall as Mega Aggron's Filter. Second, Steelix is Steel/Ground, which comes with a few more weaknesses and few extra resistances. Third, it's slow; base 30 outpaces pretty much nothing outside of Trick Room. And finally, it just doesn't have the movepool to be a good wall, with no recovery options and few support moves. Instead, it works best as a sort of mix between a hazard setter and a physical tank.
Gen 7 hasn't really given Mega Steelix any significant new options, so basically you're just recycling the old strategy:
Ol' Ironsides
Ability: Sand Force (Sturdy on Steelix)
EVs and Nature: 252 HP / 120 Def / 136 SpD, Impish Nature
Item: Steelixite
- Stealth Rock
- Heavy Slam
- Earthquake / Stone Edge
- Roar / Toxic / Stone Edge
It should be noted that this set assumes Mega Steelix would still be facing off against UU threats in Gen 7, which I doubt it will. That said, the EV and Nature spread here is designed to allow you to survive a Sheer Force Life Orb Flare Blitz from Darmanitan after Stealth Rocks, which fortunately Mega Steelix doesn't mind too much. If Darmanitan will not be in the same arena, you can put more investment into SpD or Atk, depending on how you build your moveset. Chances are Mega Steelix will get at least one turn, so Stealth Rocks will be your first move. Heavy Slam is an obvious choice given Mega Steelix's insane weight. Earthquake is another STAB and hits other Steel-types hard. For the last slot, you'll usually want a support move; Steelix makes for a fine phaser with Roar and Toxic will let you bring down opposing physical walls. Stone Edge can be used in the last two slots optionally, either to broaden your coverage or to hit aerial targets. The basic strategy is to bring Mega Steelix into a physical attacker and either whittle it down, kill it outright, or force it to switch.
This set is easy to circumvent, unfortunately; just bring in a special attacker with super effective attacks and that's it. Alternatively, burn it, Mega Steelix has no recovery and will go down easily enough. Other options for Meag Steelix include Rock Slide for accuracy and PP or the Curse/Gyro Ball set that regular Steelix uses. However, be aware that without Leftovers the Curse/Gyro Ball set loses much of its effectiveness.