LGPE OU is surprisingly fun, and Melmetal is big in the Meta, so here is the set I use:
- Adamant Nature
- Double Iron Bash
- Thunder Wave/Ice Punch/Rock Slide
Double Iron Bash is an essential damaging move for Melmetal. Earthquake handles common Steel, Electric, Poison, and Fire threats like Partner Pikachu, Alolan Muk, Mega Charizard X, and Alolan Dugtrio. Because of the lack of status recovery, Steel types, Poison types, Fire types, Electric Types, Rest, and Partner Eevee's Sparkly Swirl are all, so a big part of the Meta is avoiding and healing Status conditions. Melmetal fits well into this by dealing out Paralysis with Thunder Wave, Heals it's bulky build with Rest, and is immune to Poison. Ice Punch/Rock Slide hits Zapdos and a few other Pokémon better than its other moves, and prevents being walled by them.
Hope I Helped! :P