Item: Oran Berry/Sitrus Berry/Focus Sash/Leftovers (depending on choice of fourth move, see below)
EVs: 4 HP/252 Atk/252 Speed
- Spirit Break
- Foul Play
- Trick/Thief/Protect/Stomping Tantrum/False Surrender/Play Rough/Dark Pulse/Dazzling Gleam
Yes, Prankster is the best ability for Grimmsnarl, and yes, it is essential for how we're using this beast, but if you build Grimmsnarl as a prankster build (meaning 3 or all 4 of its moves are status moves that make use of its ability), then you are doing this creature a massive disservice. It has a 120 base attack. That is higher than Mega Mawile before Huge Power kicks in (105), higher than Mimikyu (90), and even higher than Azumarill even after Huge Power kicks in (making it base 100), making it the best physical attacking fairy type to date, not counting legendaries (Zacian beats it out, obviously). So we're going to run with this in what I feel is the best way to use Grimmsnarl - the counter to pranksters, walls, and anyone else who relies heavily on setup.
Being a dark type, you automatically shut down any other pranksters. However, with that poor 60 base speed, even with 252 EVs in it, pretty much every other prankster in the game, with the exception of Sableye, outspeeds you. This is why we went with Prankster over the other two abilities. You can get your taunt off before they get their Substitute off, enabling you to shut down whatever plan they have. Likewise, this lets you stop walls before they get too deeply set up with things like Stockpile, Amnesia, Iron Defense, or Ingrain. It also lets you stop members of their team from setting up hazards like Spikes and Stealth Rocks while also preventing them from using Defog to get rid of any hazards you've setup. Basically, this is your primary role on any team, shutting down all but their pure attackers.
Spirit Break is Grimmsnarl's signature move, and it lets you attack those special attackers that wouldn't suffer as much from Foul Play, while also weakening their main attack stat. It's also your STAB Fairy move, useful for when coming against another Grimmsnarl (which is why we went with the EVs in Speed over something more defensive, to counter their attempts to counter you).
Foul Play is STAB Dark, and is great for use against powerful physical attackers, especially ones who have higher attack than you.
The final move is up to you. Trick and Thief are great options for taking Leftovers away from opponent walls, which is when you would pick to run an Oran Berry to really make them salty. If using Thief instead of Trick, any other kind of berry or even a Focus Sash would be a good alternative. Something that gets used up. Otherwise, I would suggest running leftovers. Protect is a general fourth pick to use for trying to sus out what type of set your opponent may be running on whatever they have on the field. These are your more pranksterish choices, but there's many great offensive choices as well.
Stomping Tantrum is a great coverage move for two of your three weaknesses, dealing with both Steel and Poison types. False Surrender is a great dark STAB move for using against those who are weak to dark type but may not be great physical attackers (such as most Psychic types). And Play Rough is a higher power fairy STAB move than Spirit Break that has a chance of lowering the opponent's attack. I wouldn't pick it as an alternative to Spirit Break, as Spirit Break is a guaranteed Special Attack drop, while Play Rough is only a 10% chance for dropping the physical side of things. Other great alternatives for a fourth move are Dark Pulse and Dazzling Gleam for countering physical walls that aren't so great against special attackers. While Grimmsnarl is clearly a superior physical attacker, that 95 base special attack is nothing to sneeze at, and a great option for making this a wall breaker.