You have to have an odd keystone, go underground, and talk to at least 32 people down there (it can be the same person, but each time you talk to him/her, they will have to go back to the surface, come back down, and talk again to count as 2). Once you have done that, there should be a spiritomb rock in the southern part of the route south of Solaceon town (where the daycare couple are). Use the odd keystone on it, and you will be able to catch spiritomb. Note: if you fail to catch it, you will have to talk to 32 more people underground and get another odd keystone.
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