Excadrill would be quite good. Its STAB Ground moves are super effective against Skuntank, it's immune to Skuntank's Poison moves, and it resists Malamar's Psychic moves.
Excadrill @ anything
Ability: doesn't matter
- Earthquake (Level 52/TR)
- Iron Head (TR)
- X-Scissor (TR)
- Brick Break / Swords Dance (TM found on Route 8 [Brick Break] / Level 40 [Swords Dance])
With this set, it can cover every single one of Piers' Pokemon super effectively. A second good Pokemon would be a Bug type, which resists Scrafty's Fighting STAB and gets STAB on super effective Bug moves itself. I would recommend Ribombee, as it resists all Dark STAB, and can hit both Malamar and Scrafty for 4x damage with its STAB, plus Obstagoon for 2x damage.
Ribombee @ anything
Ability: Shield Dust
- Bug Buzz / Pollen Puff (Level 56 [Bug Buzz]/Learned upon evolution [Pollen Puff])
- Dazzling Gleam (Level 48/TR)
- Psychic (TR)
- Energy Ball (TR)
Hope I helped!