Here's a Gen 7 1v1 set for Marshadow that I came up with (along with a small improvement from a kind person).
Marshadow @ Life Orb / Marshadium Z
Ability: Technician
EVs: 4 HP / 252 Atk / 252 Spe
Lonely Nature
- Bulk Up
- Close Combat
- Spectral Thief
- Shadow Sneak