I have two movesets, because why not.
Melmetal @ Assault Vest
Ability: Iron Fist
EVs: 126 HP / 252 Atk / 4 Def / 126 SpD
Adamant / Brave Nature
- Earthquake
- Double Iron Bash
- Thunder Punch
- Darkest Lariat
Explanations:
Natures: Brave is only if you are running a trick room Melmetal, otherwise use Adamant.
Evs: People are going to try to exploit his relatively low SpD, so he needs some bulk in that. HP Evs are always nice and help more than defensive Evs. the Atk Evs are to support his already massive base 143 attack stat.
Moveset: You could run Body Press, but I prefer Darkest Lariat for better coverage. Thunder Punch is necessary because of the ability Iron Fist boosting it's base power.
Item: Assault Vest helps support his SpD even more, but you could run Choice Band, but I don't think it's needed.
Melmetal @ Occa Berry
Ability: Iron Fist
EVs: 4 Atk / 252 Def / 252 SpD
Adamant / Impish Nature
- Acid Armor
- Body Press
- Rock Slide
- Double Iron Bash
Explanations:
Nature: Impish is the better one, because this set relies on defense mostly, but you can use Adamant if you want.
Evs: You need to be able to tank a few hits to use Acid Armor, and those extra Evs will support that.
Moveset: Acid Armor to raise Defense, Body Press to utilize those defense raises. Rock Slide and Double Iron Bash are to flinch.
Item: Occa Berry to tank a hit well to set up Acid Armors.
Hope This Helps!