This is a good moveset for Spiritomb I would know because I LOVE ghost type pokemon, I haven't lost since with this move set so here it is:
Hypnosis: Gives you time to attack or raise stats, or heal pokemon.
Dream Eater: Good for gaining HP, works well with Hypnosis
Impison: This is a HUGE advancement, since Spirtomb is a ghost type and ghost type moves are super effective on other ghost types this allows you to cancel out the opponents pokemon's same moves that have the same moves as you.
Sucker Punch: STAB, good power, always attacks first.
Hope this helps!!!:D