I used:
Item: Eviolite
Nature: Impish (+ Def, - SpA)
EV Spread: 252 Atk/212 Def/44 SpD
Moveset: Swords Dance, Shadow Sneak, Shadow Claw, Iron Head.
Keeping in mind that if the opponent leads with Mega Gengar, switch out to a Normal type or stay and do Shadow Sneak and hope for a critical...