Moves
Flame Charge is a vital move, as it doubles Numel's Speed stat thanks to Simple while dealing some chip damage, allowing it to outspeed the majority of the metagame. Fire Blast is a powerful STAB move that is able to OHKO Foongus after Stealth Rock before a boost, and after a Growth boost, only Fire-resistant Pokemon and very bulky Pokemon like Spritzee aren't OHKOed by it. Earth Power provides great and accurate neutral coverage with Fire Blast and nails Rock-, Electric-, and Fire-types such as Onix, Elekid, and Ponyta while giving Numel a way to damage Water-types, though it needs a boost from Growth or Groundium Z to do serious damage to them. Growth is used to double Numel's Special Attack stat, giving Numel some much-needed firepower for non-Groundium Z sets and, when combined with a Flame Charge boost, making Numel a terrifying sweeper. If you're running Groundium Z, Stealth Rock is a nice option, as the Special Attack boost from Growth isn't really needed, allowing Numel to set entry hazards if they aren't on the field already.
Set Details
Simple is mandatory for this set to function, since it doubles all the stat increases from Flame Charge and Growth. Maximum Special Attack investment with a Modest nature is used to make Numel deal as much damage as possible. 236 Speed EVs are used so that Numel outspeeds 17 Speed Choice Scarf users after a Flame Charge, with the remaining EVs going into HP to boost it by one point. Eviolite is used to give Numel more bulk so it can more easily tank attacks and set up on more Pokemon, while Berry Juice provides recovery Numel much desires to set up on Pokemon it walls. Groundium Z transforms Earth Powerinto a 175-Base Power Tectonic Rage that lets Numel nuke a threat when performing a sweep; for example, it is capable of OHKOing defensive Staryu after Stealth Rock damage and dealing significant damage to bulky Pokemon like Spritzee. A Timid nature could be run to outspeed Choice Scarf Doduo and Gastly, which could otherwise revenge kill Numel after a Flame Charge boost, but the loss of power isn't very favorable.
Usage Tips
Numel functions as a late-game sweeper, so it is best to remove any checks and counters, especially Aqua Jet users like Corphish and Tirtouga, before sending Numel in. It should also be switched in on Pokemon it threatens out or Pokemon that can't do much damage to it, like Ferroseed, Magnemite, and Pumpkaboo-XL, to let Numel more easily set up with Flame Charge or Growthor get Stealth Rock up. Flame Charge should generally be used before Growth, as Numel is quite slow before a boost and will likely be outsped and KOed by a switch-in otherwise. Groundium Z sets can work as a lure to Staryu wanting to switch in and trying to remove entry hazards and check Numel, OHKOing them after Stealth Rock damage. It is also useful for taking out a very bulky Pokemon such as Spritzee or Timburr during a sweep. On sets that lack Groundium Z, Numel should tank a hit to set up either Growth or Flame Charge, as it wants both boosts to be able to safely sweep.
Team Options
Grass-types such as Foongus and Snivy can deal with the Water- and Ground-types that otherwise check Numel. Even if Numel decides to run Stealth Rock, entry hazard support from Onix, Dwebble, or Ferroseed is still appreciated, as it means Numel doesn't have to risk setting up Stealth Rock itself, giving it an easier time sweeping. Ferroseed is an especially great partner for Numel, as its Grass typing lets it come in on common Water-types and set up entry hazards on them. Knock Off users like Mienfoo, Timburr, and Vullaby soften up bulkier Pokemon like Spritzee and opposing Vullaby, making it easier for Numel to break through them. Mienfoo can even lure in Pokemon Numel beats, such as Pumpkaboo-XL and Foongus, and pivot Numel in with U-turn, making it a great teammate. Memento users such as Diglett and Cottonee lower the foe's offenses while safely bringing Numel in, providing it with more setup opportunities. Staryu makes for a great partner, as it can deal with Water- and Ground-types with its coverage moves and is able to remove Sticky Web, which otherwise halves Numel's Speed, thus requiring an extra turn of setup. It is also able to lure in Ferroseed and Pumpkaboo-XL, Pokemon that Numel can set up on. Alolan Vulpix provides Numel with much-needed bulk and survivability via Aurora Veil support, making it a lot easier for Numel to set up.