The fastest way to farm Pokemetal
The fastest way to stockpile Pokemetal is the Minecraft-style dig-and-Rollout method on the Dragonite Dream Island, not surface mining.
Pokemetal itself is a refined material — you get it by smelting Pokemetal Fragments in a Smelting Furnace with a Burn-specialty Pokémon. Fragments come from Pokemetal ore blocks, which are tough enough that Rock Smash needs to be powered up first by eating a Hamburger Steak before it can break them. Fragment blocks turn up underground in the Withered Wasteland mine (north entrance, behind a gate that needs an upgraded Cut) and in caves on the eastern side of Sparkling Skylands.
The fastest known method skips those static deposits entirely and instead farms a Dream Island. Every day, giving Drifloon a Dragonite doll ferries you to a Dream Island that players have found to be the densest Pokemetal source in the game. From the beach near your arrival point, dig straight down six blocks with Rock Smash, then switch to Rollout and drive straight lines through that layer — it's thick with Pokemetal ore, similar to classic branch-mining in other block-building games. One widely shared run reported over 1,200 Pokemetal fragments in about 90 minutes using this technique.
If you'd rather not mine, a Pokémon with the Trade specialty can hand over Pokemetal once you have a working Cash Register or fully rebuilt Pokémon Center — you pay in items worth a set value (crops are a cheap way to hit that total), which turns spare farm produce into rare metal without touching a single ore block.
Related: Pokemetal, Pokemetal Fragment
Regions: Withered Wasteland, Sparkling Skylands
Sources disagree on the exact mechanics of the Dream Island trick and its yield; the six-block depth and 1,200-in-90-minutes figure come from a single widely-reported player run and aren't independently re-verified by a second source.