GOBLINS
Putting aside the fact that this guy would probably go into murder mode if he ever met Grimmsnarl, this grotesque fairy has found a niche home in the higher tiers of competitive play namely because it is one of the few users of the Prankster ability that can utilize more offensive sets thanks to excellent offensive and defensive typing, base 120 attack, a large movepool, and just enough support options that can take advantage of Prankster. Granted Grimmsnarl is still not quite as terrifying as the goblins from "Goblin Slayer". The agro warrior seen above can easily catch up to him no thanks to low base 60 speed when it is not using a status move, and 95 65 75 defenses aren't so high that Grimmsnarl can take a dagger from Goblin Slayer. Still, face off against Grimmsnarl at your own peril with a plan in mind and fully aware of his capabilities, or well...you already know what will happen:
Sleep Sweeper
Ability: Prankster
Item: Leftovers
Nature: Adamant
EVs: (HP 252) (Attack 252) (Speed 4)
Moves:
Darkest Lariat
Play Rough/Spirit Break
Rest
Sleep Talk
Set Details:
Grimmsnarl's Prankster ability only gives +1 priority to its status moves, but this set extends the privileges to its offensive moves thanks to Sleep Talk technically being a status move, thus giving you priority to your attacks when asleep after using Rest (Which also has priority). Darkest Lariat is useful for bypassing enemy defense boosts or your attack drops, and while Play Rough is the stronger of its two physical Fairy attacks, Spirit Break is 100% accurate and punishes special attackers with a special attack drop each time it hits. Invest in attack and bulk, and slap on Leftovers to increase survivability.
Screen Setter
Ability: Prankster
Item: Light Clay
Nature: Adamant
EVs: (HP 252) (Attack 252) (Speed 4)
Moves:
Reflect
Light Screen
Darkest Lariat/Sucker Punch
Spirit Break/Drain Punch
Set Details:
Priority wall setting is a godsend as any Klefki players can attest, and unlike the key ring, Grimmsnarl can apply offensive pressure. Set up a corresponding wall against a physical or special attacker followed up by the other one, and thanks to Light Clay they will both stick around for a while and allow your teammates to take advantage of them. And of course unload your STABs once your screens are up. This time Sucker Punch is an option as well, allowing you to bypass your low speed without reliance on Prankster provided you can anticipate an opponent's attack, and you can substitute Spirit Break with Drain Punch for improved survivability and wider coverage if you prefer.
Spark and Slug
Ability: Prankster
Item: Leftovers
Nature: Adamant
EVs: (HP 252) (Attack 252) (Speed 4)
Moves:
Thunder Wave
Bulk Up/Drain Punch
Darkest Lariat/Sucker Punch
Play Rough/Spirit Break
Set Details
Since speed is not Grimmsnarl's forte outside of status moves, Thunder Wave rectifies that with priority paralysis that also offers team support. Bulk Up can be used to accumulate offensive boosts, but Drain Punch offers added coverage and HP recovery, and of course you still have your STAB options to take down your opponent.
Other Options:
Nasty Plot: Base 95 special attack means a special attacking set is viable with Grimmsnarl using moves like Dark Pulse, Dazzling Gleam, Focus Blast, and Burning Jealousy, but the base 60 speed without its priority status moves keep it from fully utilizing the set without Thunder Wave.
Swagger/Flatter: Another status option to mess with your opponent, but it is not as reliable and can backfire if luck is not on your side, and the former is banned from competitive play anyways.
Metronome: Speaking of luck, since Metronome is technically a status move it gets priority with Prankster. If you are lucky you will get your hands on a useful offensive move randomly plucked from the attackdex, but if you are unlucky you could draw something useless or even detrimental. Still fun to use though, and hilarious when you can beat your opponent with it (Cue salty competitive players)
So show no mercy out there, let loose your inner villain, and brutalize your opponent, for you are in control of a goblin champion.