Battle Moveset

A Good Moveset for Grumpig would have to be this:

Attacks:

Psychic
Calm Mind
Substitute
Thunderpunch / Ice Punch / Fire Punch

Items Attached:

Leftovers

Preferred Nature:

Calm

Preferred Trait:

Thick Fat

Strategy Using Grumpig

As with most Calm Mind-Substitute Pokémon the strategy revolves around setting up an invincible Substitute to the point where if a physical sweeper comes into play it'll be OHKOed but no special sweeper can break through the Sub. While the strategy is well known and on Grumpig it's practically a given that this strategy will be used when timed right there isn't much your opponent can do about it. Grumpig can also fill in the role of a special wall, with average HP and above average special defence as well as a handful of special resistances (Psychic and with Thick Fat, Fire and Ice) it can fill the role suitably. It's physical defence leaves a lot to be desired however it's special attack can pack a decent punch and few of the common UU physical sweepers have a resistance to Psychic or enough special defence to shrug it off.

EV Corner:

EVs: 140 HP / 176 Spd / 136 SAtk / 56 SDef
Calm Nature (+SDef, -Atk)

336 HP for solid leftover recovery, 250 Special Attack and 297 Special Defence at base prior to Calm Minds and enough speed to outrun Pokémon from the Base Speed 70 Category (such as Magneton, Solrock and Lunatone) who run max speed without a nature boost.

Other Optional Sets

Grumpig @ Choice Band
Trait: Thick Fat
EVs: 140 HP / 176 Spd / 136 SAtk / 56 SDef
Calm Nature (+SDef, -Atk)
- Psychic
- Calm Mind
- Trick
- Substitute / Thunderpunch / Fire Punch / Ice Punch

Less predictable variation of the main set. Trick-Band when used properly will cripple a special sweeper or tank however the risk is a sacrificing versatility and the potential of passing the Choice Band to something that can use it effectively.

Other Optional Moves

Light Screen, Counter, Magic Coat, Taunt, Shadow Ball, Focus Punch, Hidden Power [Dark]. Light Screen helps in sponging special attacks and can be passed by switching but Calm Mind sweeping is usually preferred. Counter can potentially KO a physical sweeper but that hurts Grumpig's ability to sponge special attacks and doesn't go well with it's mediocre speed stat. Magic Coat is just a "fun" move and is hard to use effectively. Taunt is a good move for a stat-up sweeper but not worth sacrificing versatility. Shadow Ball and Focus Punch could be used with Trick in a novelty/surprise set, the low defence of many Psychic's gives Choice Band Shadow Ball some potential and Focus Punch is a powerful enough move to leave a dent even with Grumpig's pitiful attack stat. Hidden Power [Dark] can counter other Psychics who could easily wall you.

Strategy Against Grumpig

It has weak defence and mediocre speed so fast physical sweepers can capitalise once in play. Thunder Wave severely cripples it's ability to sweep and Toxic or Will-o-Wisp will leave it unable to stall. The majority of Dark Types counter it with ease and any Psychic type with appropriate attacks to handle Psychic types provide suitable counters. Most of the fast UU Choice Banders can KO it or close enough.

Contest Moveset

A good Contest Moveset for Grumpig would have to be this for the Smart Contest best with Calm, Gentle, Careful, Sassy Nature:

Attacks:

Calm Mind
Psychic
Shadow Ball
Confuse Ray

Items Attached:

Green Scarf

Strategy Using Grumpig

If you do the attacks in this following order you should have very few problems; 1st - Calm mind, 2nd - Psychic, 3rd - Calm mind, 4th - Psychic/Shadow ball, 5th - Confuse ray

Locations in Games

Ruby/Sapphire/Emerald Evolve Spoink Colosseum/XD Trade from RSE Fire Red/Leaf Green Trade from RSE

Animé Appearences

Grumpig has had a few Anime Appearences. In them, it was used by a co-ordinator in her Contest Match against May, in which, it lost: Episode 366: PokéBlock, Stock & Berry! Episode 367: Lessons in Lilycove!