For a mono-fire team, Torkoal is my most recommended tank. I'd use the set Hex posted:
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Torkoal @ Chesto Berry
Trait: White Smoke
EVs: 252 HP / 160 SAtk / 96 SDef
Modest Nature (+SAtk, -Atk)
- Earth Power
- Eruption
- Amnesia
- Rest
Unless you want to set sand with Tyranitar, Scrafty and Drapion are your best dark-type tank choices. I personally prefer Scrafty, with a set that looks something like Mew's:
Scrafty (M) @ Leftovers
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Trait: Shed Skin
EVs: 252 HP / 4 Atk / 252 SDef
Careful Nature (+SDef, -SAtk)
- Drain Punch
- Bulk Up
- Rest
- Ice Punch