Usually if you're going to use Confuse Ray, its not a good idea to combo that with Will o' Wisp because you want to keep the opponent's Attack stat high so it'll do alot of damage to themselves. I say go with Toxic instead. Its too bad he can't learn Mean Look, or else he would've made one heck of a great tank.

I do like the idea of Pain Split on him, but his Speed is so slow that he might not even get a chance to actually use it. I guess you should playtest a bit Dark and see what works for you. But for sure, use Spiritomb as a tank/annoyer. He's not made for sweeping