Ok Haunter is tricky.
It's tanky stats aren't good enough for evolite to pack a punch, but something it can do it ghost a Gengar.
IN 1v1, bassically you build it liek Gengar, and use it after gengar has elminated some of the threats, and hope haunter can do the same job, or at least instill the fear of it.I REALLY ADVISE choice scarf for this one)
IN doubles I'll post once I get home.
EDIT: HOME!
So in doubles, Again, no reason to run haunter instead of Gengar sadly, but again, you can ghost him. Except this time, you have one as the sweep, and one as a status inducer. Traps are fairly uncommon in doubles, so you can worry aboptu them a biut less. And with a choice scarf on haunter, and swapping and status inducing, then bailing. Would work best with a poke that is tanky with regenerator.
But that's about it since, unlike some pre evo's, we was intended to be less effective than his future self. But these stragiteys in and of themselves aren't all that bad if you can out think your oppoent.