To catch Spiritomb (without using Action Replay), you need to talk to 32 different people in the Sinnoh Underground. (Not including Hikers, sorry!) You can do this by having a friend keep exiting and respawning in the same spot 32 times. You now need an Odd Keystone, so go to TwinLeaf Town. Surf there, and use the ItemsRadar. It should be on the left side of the pond. Then, go south of Solaceon Town and go to the little stone well, south of the Pokemon Cemetery. It will ask you if you want to insert the Odd Keystone. Say yes, and Spiritomb will appear. It is level 25, so bring Pokemon of that level. You only get one chance. To get another chance, you will need a second Odd Keystone and need to talk to 32 more people in the Sinnoh Underground.
I hope this helps!
