Yeah, Spiritomb's HP is too low to not pay attention to it.
So max out HP to begin with, and then either max out one of his defenses, or split it up between the two of them (if you want a powerhouse single wall, or a durable double wall). The nature could also go towards the defense you want to boost. I prefer to go towards a -Speed nature, but you can go with -Atk/-SpAtk if you really want to, depending on the moveset you choose. I, personally, went with Sassy.
As for the moveset... there are two I found that seem quite effective:
Spiritomb @ Leftovers
- Dark Pulse/Pursuit
- Pain Split
- Will-O-Wisp
- Hypnosis
I tend to lean towards Dark Pulse, but with Pursuit, Spiritomb makes the greatest Alakazam counter EVER. He's immune to Zam's two most powerful moves (Psychic and Focus Blast) and takes mediocre damage from Shadow Ball. When he flees, Pursuit him for a quick KO. This moveset also makes a decent lead; it's my lead, actually. XD
The second moveset:
Spiritomb @ Leftovers
- Sucker Punch
- Will-O-Wisp
- Pain Split
- Taunt
Taunt-Punching Spiritomb; a set Blissey truly fears. It requires a bit of prediction, since Blissey usually outruns him for a Thunder Wave, but if you can Taunt her, then Burn her, she's free game for you. Sucker Punch does decent damage, since it's physical, and with her enormous HP, Pain Split does good damage even if Spiritomb's HP is full! All this while Will-O-Wisp slowly wipes down her HP, and because of the Taunt, Blissey can't 1.) Aromatherapy to heal her Burn, or 2.) Softboil for a stall war. It should be noted this would work most effectively if Blissey was the last Pokemon for your opponent, which happens more than you would think.
