Melmetal immediately draws comparisons to Rhyperior with its STAT loadouts. Incredible HP, ATK and DEF, poor Special Attack and Special Defense and atrocious speed. Melmetal stands out from Rhperior thanks to its superior typing and utterly bonkers STAB in Double Iron Bash, which is boosted by Iron Fist, making it an effective base 142 power move before STAB. However, Steel is a poor offensive type and you will never proc the Flinch unless you have Trick Room up.
Also, don't use Melmetal's Gigantamax. You actually receive a minor drop in power and the Torment effect is not worth it as opposed to the Def boost vanilla Dynamax provides.
Mercury Mercenary
Melmetal
Ability: Iron (hah!) Fist
Item: Assault Vest
-Double Iron Bash
-Earthquake
-Ice Punch
-Thunder Punch
EVs: 252 HP / 116 ATK / 134 SDEF
Nature: Adamant (+ATK, -SATK)
Melmetals ludricrous HP and DEF make it a decent recipient of Assault Vest to patch up its Achilles heel of its SDEF. Not the best due to the lack of recovery, but you could do worse. Double Iron Bash is nuts as a STAB move. Base 142 move before STAB that busts through Disguise, Focus Sash, Sturdy and Substitute? Most pokémon would kill for a STAB that good. Earthquake, as noted on Copperajah's cover provides great coverage vs Steel's Resists (Steel, Fire, Electric). Thunder Punch takes care of Flying Steel types and the Water types that resist Double Iron Bash while Ice Punch slays the many many quadruple Ice weak threats in the game. EVs are to maximize bulk as much as possible while still giving some oomph to attack.
Steel and Ground also synergize well for a Defensive Dynamax sweeper. Pair this with Wish Support to push Melmetal to near immovable object Territory.