Spiritomb is a very unique pokemon being part ghost and dark like Sableeye it has no weaknesses.
Step 1 Find a odd keystone from underground or recive from hidden man on sinnoh.
Step 2 take the odd keystone to a spirit well on route 209 and place it inside. This will activate the event to getting Spiritomb.
Step 3 Talk to 32 people or one person 32 times. (These Dont include the Hikers you see) and if you do the same one over again you must enter and exit everytime.
Step 4 Now with every thing above completed you can go back to the well and catch him but he is very low leveled say between 10 and 20. you may do this multiple times.