Shadow Cacturne

Shadow Cacturne definitely exists. It can pull its weight as a Dark Type attacker, hitting for a solid amount of damage, and can be useful in raids where its Grass typing comes in handy in particular. However, it's really not worth the steep investment to max out for most players.

Best PvE Offensive Moveset

• You almost always want Sucker Punch in any situation that you’d use Shadow Cacturne.
• Sand Attack and Poison Jab are good, but not really on Shadow Cacturne. 
• Dark Pulse shares typing with Sucker Punch to make a decent Dark-type moveset.
• Payback is a step down.
• Grass Knot is one of the better generally available charge moves in the game, compensating for lack of fast move synergy.
• Trailblaze is basically Grass Knot but worse.
• Dynamic Punch is great, but best left to Fighting-types.

Best PvE Defensive Moveset

• Poison Jab slightly outdamages Sucker Punch, but the latter gains energy faster.
• Sand Atack falls behind on Defense.
• Dark Pulse and Grass Knot both get STAB, but are more likely to be resisted by many attackers.
• Trailblaze is basically Grass Knot but worse.
• Dynamic Punch is less likely to be resisted and has wide coverage. 
• Payback is undesirable for defense due to it being a 1-bar move. 

Sucker Punch is the preferred Fast Attack, boasting STAB and the highest damage output compared to its other Fast Attacks while still having above energy generation.

Trailblaze is Cacturne's lowest cost Charged Attack; it also benefits from STAB, does decent damage, and boosts Cacturne's Attack after each use. Dark Pulse can help increase the reliability of Cacturne's Dark-type damage, while Dynamic Punch offers improved coverage against Steel-, Normal-, Ice-, and Dark-type Pokemon. Payback is too expensive.