If you bring in weather effects (hail or sandstorm), then that
will damage it. Also, burns or poison will damage it.
OR
Simply use Miltank + Scrappy + Brick Break + Choice Band =
2/3HKO. Spiritomb can't do sh*t to Miltank with Shadow Sneak or
anything else.
OR
Focus Sash/Choice Band + Mold Breaking Rampardos/Pinsir,
Spiritomb can't do anything except be sashed (where Sandstorm comes
in).
OR
Jirachi Doom Desire + Skarmbliss Wish combo on it if it has a
Wondertomb party.
OR
Breloom comes in and maximize Spore usage (since the opponent
uses Wondertomb, technically no clause is in effect as if he can
cheat, you can too). After, a Pokemon that can hit it comes in and
pwns it.
OR
Use a fighting Pokemon like Machamp + No Guard, use Foresight
and kill it with a super effective Dynamic Punch.
OR
Use a Pokemon with Fire Fang; like Entei.
OR
Use your own Wondertomb, what can he do?
OR
Use Gastro Acid or Worry Seed, and go in for the kill.
OR
Go up to the challenger and kick his balls and take a dump all
over his DS.
NOTES:
1. Set Stealth Rock.
2. Set Sandstorm (with TTar if preferred since it has Fire
Fang)