Steelix @ Steelixite
Ability: Sturdy
EVs: 252 HP / 4 Atk / 252 SpD
Careful / Sassy Nature
- Curse
- Earthquake
- Heavy Slam / Gyro Ball
- Toxic
note: Heavy Slam is preferred mostly due to PP, but obviously Gyro Ball is viable.
Curse + Toxic Mega Steelix serves as a late-game win condition with mid-game utility. The problem with Curse + Rest Mega Steelix is that if the opponent has one of its answers (e.g. Slowking, Quag, non-scarf Moltres) then Mega Steelix really won't do much especially as these answers all have recovery. Toxic + Protect Mega Steelix has similar problems but in different Pokemon: it gives endless free turns to Immunity Gligar, Bronzong, or physically defensive Amoonguss.
Curse + Toxic Mega Steelix offers a solution to the common switchins of both Curse Steelix and Toxic + Protect Steelix. Bulky waters, Spiritomb, and Moltres don't appreciate Toxic. Immunity Gligar, Bronzong, and physically defensive Amoonguss don't mind Toxic, but they're set up fodder. Steelix's matchup against clerics is similar: it flatout beats Aromatisse/Audino, and nearly OHKOs Lanturn (it could get burned though).
Things that are immune to Toxic that are Curse fodder include Escavalier, Registeel, Bronzong, Immunity Gligar, Amoonguss, Drapion, and physical Cobalion. Specs HP Fire Magneton does 50% max, but is OHKO'd by Earthquake. Durant isn't exactly fodder, but it doesn't beat Steelix. Focus Blast Cobalion probably beats this set, but that's very rare.
This set requires very little support other than a dedicated Moltres switchin. Wish and cleric support are helpful, but not needed.