The savior of millions of raids, Grimmsnarl has been a staple for many raids and all of the doubles formats it has been allowed in for being simply one of the best Prankster abusers in the game. Most of the time, this is in service of setting up Reflect and Light Screen from the word Go, making Grimmsnarl's teammates incredibly hard to break through, but Grimmsnarl's Prankster antics go far beyond that. Taunt to stop opposing setup, Thunder Wave or Trick Lagging Tail of all things to slow a target down, Fake Tears for a damage buff, Misty Terrain to disrupt the likes of Rillaboom, Indeedee, and Miraidon while denying Spore, to even priority Parting Shot to sink an opponent's attack even further while pivoting Grimmsnarl out to an ally waiting in the wings. Even without Prankster, Grimmsnarl is packing support, with the doubles staple in Fake Out as well as its once signature Spirit Break to further sink an opponent's offenses. Its to the point that one forgets that Grimmsnarl can be threatening offensively. Base 120 Attack is nothing to scoff at, especially with the offensively excellent STABs in Fairy and Dark Even Grimmsnarl's typing serves it well in it support antics, blocking opposing Prankster user's Taunts while threatening any Prankster immune Dark type with its own Fairy STAB. It is to the point that Grimmsnarl is one of the biggest sufferers of Four Moveslot Syndrome, unable to fit everything it wants into a single set. This has led to many Grimmsnarl being rather passive, unable to pack as much offense as they like in both movepool and EV investment. 65/75 defenses are rather poor and only partially made up by a good base 95 HP. Dark and Fairy also react rather negatively, with Dark negating several of Fairy's resists, giving Grimmsnarl few if any resistances to speak of. This has not prevented Grimmsnarl from being a reliable and versatile threat that should be expected on the field of battle.
Captain Goblin
Grimmsnarl
-Reflect
-Light Screen
-Thunder Wave
-Spirit Break
Ability: Prankster
Item Attached: Light Clay
EVs and Nature:
252 HP / 4 Atk / 132 Def / 116 SpD / 4 Spe
Careful Nature
Tera Type: Ghost
Note that this set is far from emblematic of everything Grimmsnarl can do, this is just by far the most common moves on it. Reflect and Light Screen are near staples on Grimmsnarl given of three Prankster users that get both moves (the others being Sableye and Meowstic-M) it is by far the bulkiest, and thus able to survive to set both Screens as well as re-establish them if need be. Thunder Wave is the most common form of Speed control on Grimmsnarl and as Thundurus has proven, Prankster Thunder Wave is an incredible move, immediately and permanently sapping a target's Speed while have the potential to force the target to miss turns, a dangerous condition in the fast paced doubles format. Spirit Break rounds out the set, giving Grimmsnarl a relatively clean out to Prankster Immune dark types while making Grimmsnarl and its allies even bulkier against Special Attacks.
EVs are to increase Grimmsnarl's bulk as much as possible with the 4 EVs in Speed to get the jump on other Grimmsnarl. With the given Special Defense EVs and Careful nature, Grimmsnarl is guaranteed to survive itemless Gholdengo's Make It Rain before Light Screen as well as Timid (non Specs) Flutter Mane's Moonblast. While Terastalizing Grimmsnarl is far from ideal, Tera Ghost is great in dodging Fake Out and ensuring Grimmsnarl's Screens go up.
Other Options:
Trick with Lagging Tail or Iron Ball is another speed control option that can be even more debilitating given how so many Pokémon rely on their items to function while their downsides matter less and Grimmsnarl is often still going first with Prankster.
Misty Terrain is incredibly disruptive against staples like Miraidon, Rillaboom and Indeedee-F.
Fake Out is an incredible attack, blanking an opponent's turn when Grimmsnarl switches in and with Grimmsnarl's Dark typing, Ghosts don't want to stick around.
Taunt is great in disruptive other support Pokémon as well as stop Trick Room.