They are both Dark and Ghost type Pokemon.
No unless you use forsight or have the ability scrappy and fighting moves or if the opponent is holding bull's eye and you use fighting moves.
I believe you can also use a move that turns the enemy into a Water type (Water Log l think it's called, or Soak whatever) to make them vulnerable.
(+) Yeah, Soak that's what its called.
The ability Scrappy can do the job. It makes it possible to hit ghost type pokemon with normal and fighting attacks. Other than that, foresight/odor sleuth and fighting moves or switcherooing a bull's eye.
Skillswap Kecleon's color change! then use a super-effective move on the opponents new type.
If you use foresight then a fight type move that will be supereffective.