How about Absol:
Absol aka Critical Frenzy@Scope Lens
Ability: Super Luck
Evs: 252 Atk 252Spd 4 HP
Psycho Cut (Amazing crit rate)
Sucker Punch(For that priority but night slash is good too)
Stone Edge(For coverage)
P.S. There are not many good pure dark-types but Absol is one who is ok but I bet its Mega-Evolution is awesome.
P.S.S. This might not be for double battles but it doesn't have any good status moves.
P.S.S.S. Hope I helped:)