Grimer

Stat	base	min-	min	max	max+
HP	80	N/A	24	27	N/A
Att	80	12	14	17	18
Def	50	9	11	14	15
Sp.A	40	9	10	13	14
Sp.D	50	9	11	14	15
Spe	25	8	9	12	13

[SET]
Name: Boom Boom
move 1: Poison Jab
move 2: Flamethrower / Fire Punch
move 3: Shadow Sneak / Ice Punch
move 4: Explosion
item: Life Orb
ability: Sticky Hold
nature: Adamant
evs: 196 HP / 196 Atk / 118 Def

[SET COMMENTS]

Grimer, despite his awesome attack base, doesn't have much in the way of dealing with physical walls like Gligar. Therefore, you need to either switch or Explode. With max attack and a Life Orb, Explosion will OHKO everything in Little Cup not named Aron or Shieldon, or anything with a Ghost typing. Fire Punch deals with Bronzor quite nicely if you don't feel like Exploding quite yet, and Flamethrower is a good option as well.

It's rather pleasant that Grimer gets Shadow Sneak. The Ghosts must now fear him, as he can switch in on any of their attacks, not die, and then Shadow Sneak for either the kill or a lot of damage (Gastly is the only Ghost he can actually OHKO). Staryu is threatened in the same way; even a Modest, LO Surf isn't a guaranteed OHKO (85.19% - 100.00%). On the other hand, Ice Punch is a reliable OHKO on Gligar without killing yourself. This does require a massive amount of prediction, as Earthquake is a guaranteed OHKO.

[DAMAGE CALCS]

vs. Bronzor: 80.00% - 96.00% (Fire Punch, 252 HP Bronzor)

vs. Bronzor: 56.00% - 64.00% (Flamethrower, 252 HP +nature Bronzor)

vs. Duskull: 61.90% - 71.43% (Shadow Sneak, 252 HP Duskull)

vs. Gastly: 100.00% - 115.79% (Shadow Sneak)

vs. Bronzor: 116.00% - 136.00% (Explosion, 252 HP Bronzor)

vs. Gligar: 157.69% - 184.62% (Explosion, Max HP/Def +nature Gligar)

vs. Munchlax: 233.33% - 272.73% (Explosion, Max HP +nature Munchlax)

[SET]
Name: Curser
move 1: Poison Jab/Explosion
move 2: Shadow Sneak/Ice Punch
move 3: Flamethrower/Fire Punch
move 4: Curse
item: Oran Berry
ability: Sticky Hold
nature: Brave
evs: 196 HP/196 Atk/36 Def/36 SpD
ivs: 0 Spd

[SET COMMENTS]

The Curse set faces the same legality problems the Life Orb set experiences, making it illegal to run Shadow Sneak and the punches at the same time. Curse is an egg move in both generations, and is therefore legal. The idea here is to force a switch and Curse, giving yourself some staying power. Most likely what ever comes in next won't be able to OHKO unless it packs a boosted STAB super-effective attack. Poison Jab is for STAB, yet Explosion gives the safety of a guaranteed kill.

[ILLEGAL MOVES]

For Grimer to use the elemental punches, it needs to get them from the Emerald move tutor. However, this means that Grimer will not be able to use its egg moves from D/P, the most notable being Shadow Sneak. Note that Explosion is still legal due to being a TM.

[OTHER OPTIONS]

Grimer technically has a large enough special movepool to make a decent set, but with a special attack base totaling half of the physical and no ways to handle the problems faced by the physical set, there's no reason to use it. A Choice set faces similar problems, because physical walls will be able to take even more advantage of Grimer's lack of effective options.

In terms of attacks, Grimer has a lot of untapped potential. Acid Armor can be an effective way of giving yourself some staying power. Mean Look could be interesting on the Curse set if you manage to trap something worthwhile, like a Fighting type or a Choice user. Grimer does learn Haze as an egg move, but you won't want to risk it against Gligar. However, you may be able to use it as a revenge attack.

[EVs]

Grimer doesn't have many feasible options besides maxing attack and HP. After that, the remaining EVs can be distributed between defenses. With 118 EVs remaining, either defense can get an extra 2 points, or an extra point in both.

[OPINION]

Grimer, for all its attack and HP, is a bit on the weak side. By trading one generations moves for anothers, you gain effective weapons for one set of counters, but lose your way to deal with another. It has a decent physical movepool, a way to boost its stats, and a move that OHKOs 99% of all LC pokemon, but without the speed to effectively use its options, no way to deal with the second most common tank without killing itself, or a way to use that stat-booster without worrying about switching the next turn, Grimer will never move past BL.

[COUNTERS]

I've mentioned Gligar several times as a counter. It can safely switch in on any of Grimer's attacks barring Explosion and Ice Punch, taking a max of 36% from Flamethrower and 27% from Poison Jab, and be able to outspeed and Earthquake for the OHKO. Bronzor is NOT a counter, as Earthquake will NOT OHKO and it will get beaten in speed and Flamethrowered/Fire Punched to death. Shieldon and Aron get honorable mentions: they can switch in, survive any attack, and KO with Earthquakes. It will take them more than one turn though.