Moves
Psychic is Grumpig's strongest STAB move, but Psyshock can be used as an alternative to hit some physically frailer Pokemon, such as Magmortar, Ninetales, and Cryogonal, for more damage. Focus Blast provides great neutral coverage alongside Grumpig's Psychic-type STAB move because it nails Dark- and Steel-types, such as Shiftry, Liepard, Mawile, and Klinklang. Taunt shuts down slower Pokemon that rely on status moves to beat Grumpig, such as defensive Mawile, Mega Audino, Ferroseed, and Regirock, and prevents them from taking advantage of Grumpig. Thunder Wave cripples fast offensive Pokemon, such as Lilligant, Vivillon, and Mismagius. Alternatively, Toxic whittles down bulky Pokemon that Grumpig has trouble with, such as Mesprit, Mega Audino, and Musharna. Signal Beam can be used instead of Focus Blast to hit Malamar and Psychic-types harder while still retaining super-effective coverage for Dark-types. However, without Focus Blast, Grumpig struggles to take down opposing Steel-types. Power Gem is also an option to hit primarily Charizard as well as other Fire-types such as Pyroar and Magmortar.
Set Details
Maximum HP investment is preferred to take advantage of Grumpig's great special bulk. Running 96 Speed EVs allows Grumpig to outspeed everything up to Adamant Pawniard. The remaining EVs are invested in Special Attack with a Modest nature to give Grumpig's attacks some more power. Thick Fat sets Grumpig apart from other bulky Psychic-types by providing it with invaluable Fire- and Ice-type resistances. Leftovers provides Grumpig with passive recovery throughout the match. Colbur Berry can weaken a Dark-type's STAB attack for Grumpig to give it the opportunity to retaliate with Focus Blast, Signal Beam, Thunder Wave, or Toxic. Additionally, Fighting-types that commonly carry Knock Off, such as Hariyama, cannot OHKO Grumpig if it's holding a Colbur Berry. Assault Vest is also an option that sacrifices some its utility moves for being a more consistent switch in to Fire- and Ice-types.
Usage Tips
Use Thick Fat to switch Grumpig into high-powered Fire- and Ice-type attacks from the likes of Magmortar, Vanilluxe, and Aurorus. However, do not keep Grumpig in against strong physical attackers, such as Archeops and Tauros, because they can KO Grumpig with a little prior damage. Make use of aggressive double switches to prevent Pokemon with Pursuit, such as Liepard and Skuntank, from trapping Grumpig. Grumpig should spread paralysis on opposing offensive teams to slow them down to a pace that it can keep up with, while it can defeat slower, passive Pokemon, such as Mega Audino, Regirock, and Pelipper, by incapacitating them with Taunt and whittling them down with attacks or Toxic damage.
Team Options
Offensive Grumpig fits well on balanced teams in need of a stallbreaker with the ability to switch into Fire- and Ice-types. Grass-types, such as Tangela, Vileplume, and Lilligant, appreciate Grumpig's ability to switch into Magmortar and Ice-types. Physically bulky Poison-types, such as Weezing and Garbodor, beat the physical attackers that threaten Grumpig, such as Klinklang, Samurott, and Scyther. Additionally, Grumpig provides these Poison-types with a useful Psychic-type resistance. Ground-types, such as Rhydon and Sandslash, beat Skuntank, one of the safest switch-ins to Grumpig available. Ironically, Skuntank is a great partner for Grumpig as well because it is a safe switch-in to many of the Psychic-types in the tier, such as Musharna, Xatu, and Mesprit.