Spiritomb is in a very similar situation to last week's Hatterene, where above average defenses, but Poor HP. And just like Hatterene having problems with its bulk, so does Spiritomb, only, unlike Hatterene, Spiritomb does not have a bonkers Special Attack to fall back on. This worsens Spiritomb's effective bulk, because now to do appreciable damage it has to invest in its offenses, something Hatterene did not have to worry about as much.
The move to Generation 8 was also very unkind to Spiritomb. Yes it loves its new Ghost STAB in Poltergeist, but since its Attack and Special Attack are equal, it always had the reliable Shadow Ball to fall back on. Losing Pursuit on the other hand was a major blow. With its fantastic immunities, Spiritomb was a good Pursuit Trapper, able to get free uncontested damage on many a target that it forced out. With Pursuit gone, so is Spiritomb's niche. And while Dynamax seems to be a great solution for Spiritomb, giving it the extra Bulk and Power it always wanted, Spiritomb is in many ways the archetypical Ghost/Dark type, relying on subversion and the side effects of its moves to succeed, which are completely gutted by Dynamax. And even though the side effects of Spiritomb's STAB max moves do enable a form of Pseudoramping, Ghost/Dark is a very lackluster type offensively, having nigh identical coverage and both types are walled by opposing Dark types. And to further drive the nail in, Spiritomb has no super effective coverage for opposing Dark types, leaving them a consistent problem.
Wall-A-Tomb
Spiritomb
Ability: Pressure
Item: Leftovers
-Will-o-Wisp
-Snarl
-Hex/Night Shade
-Pain Split
EVs and Nature:
252 HP / 4 DEF / 252 SDEF
Bold Nature (+DEF, -ATK)
Dedicated walling set. Will-o-Wisp and Snarl form a synergistic combo crippling any offense your opponent can bring to bear, and enabling a setup sweeper to come in and start boosting while your opponent is forced to switch out or strike with weakened attacks. Hex pairs well with Will-o-Wisp having impressive power after the boost, while Night Shade performs well irrespective of the opponents status (some Pokémon can't be burned after all). Pain Split rounds out the set, giving Spiritomb some measure of recovery.
Pressure is ability of choice here, draining PP from repeated attacks, though Infiltrator is good for Burning through Substitutes.
Other Options
Spite -Spiritomb can run spite to further speed up the clock on a move's usage, but it is very situational. Most attacks have Max PP values of 8, 16 and 24. Of those, only the 8 PP's are feasible to drain -the others have way too many uses to drain without Spiritomb being knocked out first. Ironically, this works slightly better in higher powered metagames as many Pokémon's signature attacks only have 8 PP, but Spiritomb's flaws prevent it from functioning well in those. If PP Ups didn't exist, Spite would be fantastic, completely draining 5 PP moves after one use, and 10 PP moves after 2.
RestTalk -most reliable recovery Spiritomb has, but comes at the cost of two moveslots.