Rupee-Mega (Sableye) @ Sablenite
Ability: Intimidate
EVs: 248 HP / 252 Def / 8 SpA
Bold Nature
- Shadow Ball / Dark Pulse
- Dazzling Gleam
- Calm Mind
- Recover
+1 252+ Atk Life Orb Meloetta-P Knock Off vs. 252 HP / 252+ Def Mega Sableye: 94-110 (30.9 - 36.1%) -- 55.4% chance to 3HKO
4 SpA Mega Sableye Dazzling Gleam vs. 4 HP / 0 SpD Meloetta-P: 126-150 (36.8 - 43.8%) -- guaranteed 3HKO
+1 4 SpA Mega Sableye Dazzling Gleam vs. 4 HP / 0 SpD Meloetta-P: 188-222 (54.9 - 64.9%) -- guaranteed 2HKO

Rupee (Sableye) @ Leftovers
Ability: Prankster
EVs: 252 HP / 252 Def / 4 SpD
Bold Nature
- Will-O-Wisp
- Recover
- Super Fang
- Foul Play
+1 252+ Atk Life Orb Meloetta-P Knock Off (97.5 BP) vs. 252 HP / 252+ Def Sableye: 194-229 (63.8 - 75.3%) -- guaranteed 2HKO after Leftovers recovery
+1 252+ Atk Life Orb burned Meloetta-P Knock Off (97.5 BP) vs. 252 HP / 252+ Def Sableye: 96-114 (31.5 - 37.5%) -- 0% chance to 3HKO after Leftovers recovery
+1 252+ Atk Life Orb burned Meloetta-P Knock Off vs. 252 HP / 252+ Def Sableye: 65-77 (21.3 - 25.3%) -- 0.1% chance to 4HKO
+1 0- Atk Sableye Foul Play vs. 4 HP / 0 Def Meloetta-P: 138-163 (40.3 - 47.6%) -- 55.1% chance to 2HKO after burn damage

Spirit Bomb (Spiritomb) @ Rocky Helmet
Ability: Rough Skin
EVs: 248 HP / 252 Def / 8 SpA
Bold Nature
- Roost
- Will-O-Wisp
- Dark Pulse / Shadow Ball
- Psychic
+1 252+ Atk Life Orb Meloetta-P Knock Off (97.5 BP) vs. 252 HP / 252+ Def Spiritomb: 153-181 (50.3 - 59.5%) -- guaranteed 2HKO
+1 252+ Atk Life Orb Meloetta-P Knock Off vs. 252 HP / 252+ Def Spiritomb: 103-122 (33.8 - 40.1%) -- guaranteed 3HKO
+1 252+ Atk Life Orb burned Meloetta-P Knock Off vs. 252 HP / 252+ Def Spiritomb: 51-61 (16.7 - 20%) -- possible 5HKO
4 SpA Spiritomb Psychic vs. 4 HP / 0 SpD Meloetta-P: 150-178 (43.8 - 52%) -- 14.1% chance to 2HKO

And the hard counter...

idk the situation rn (Clefable) @ Leftovers
Ability: Unaware
EVs: 252 HP / 252 Def / 4 SpA
Bold Nature
- Moonblast
- Will-O-Wisp
- Wish
- Protect
252+ Atk Life Orb Meloetta-P Extreme Speed vs. 252 HP / 252+ Def Unaware Clefable: 164-192 (41.6 - 48.7%) -- guaranteed 3HKO after Leftovers recovery
0 SpA Clefable Moonblast vs. 4 HP / 0 SpD Meloetta-P: 242-288 (70.7 - 84.2%) -- guaranteed 2HKO
Unless Melo-P runs Iron Head.
252+ Atk Life Orb Meloetta-P Iron Head vs. 252 HP / 252+ Def Unaware Clefable: 218-257 (55.3 - 65.2%) -- guaranteed 2HKO after Leftovers recovery

But if that happens, this walls it.

Magic Conch-Mega (Slowbro) @ Slowbronite
Ability: Regenerator
EVs: 252 HP / 252 Def / 4 SpA
Bold Nature
- Scald
- Will-O-Wisp
- Slack Off
- Psyshock / Thunder Wave / U-turn / Calm Mind
+1 252+ Atk Life Orb Meloetta-P Extreme Speed vs. 252 HP / 252+ Def Mega Slowbro: 133-156 (33.7 - 39.5%) -- guaranteed 3HKO
4 SpA Mega Slowbro Psyshock vs. 4 HP / 0 Def Meloetta-P: 236-282 (69 - 82.4%) -- guaranteed 2HKO