Finding Spiritomb:
1. Get the Odd Keystone: This item is found in the Lost Tower on Route 209. You'll need to use the Cut HM to access it.
2. Speak to the 30 People: There are 30 people scattered across the Sinnoh region who you need to speak to after obtaining the Odd Keystone. They are marked with a question mark on your map. Talking to them will trigger a special event.
3. Go to the Tower of Spirits: After speaking to all 30 people, return to the Tower of Spirits (on Route 209) and talk to the person inside. You'll trigger a battle with Spiritomb.
Catching Spiritomb:
* Spiritomb is level 60 and can hit hard. Use strong Pokemon, preferably ones resistant to Ghost-type moves like Dark, Ghost, or Psychic types.
* Bring plenty of Poke Balls: It will likely take many attempts to catch Spiritomb. Ultra Balls and Master Balls have the highest catch rates.
* Use status ailments: Try to inflict status ailments like Sleep or Paralyze to make it easier to catch.
* Avoid using strong attacks: Don't use attacks that might knock Spiritomb out before you have a chance to catch it.
Good luck catching Spiritomb!