I would probably do this:
Ability: Truant (no other option)
STAB, and the power gets better as the friendship raises for Return, huge power for Double-Edge but recoil damage, and big power again but shaky accuracy for Mega Kick.
Coverage against Steel and Rock types, and works extremely well with the next move.
Gets Ghost-Fighting coverage with Brick Break, and is your weapon against Ghost types.
Bulk Up/Earthquake/Rock Slide/Elemental Punch
Bulk Up can be used for boosting your attack and defense and make you an absolute monster. Earthquake, Rock Slide, or one of the Elemental Punches are all coverage options.