I have an idea

Celesteela @ Misty Seed
Ability: Beast Boost
Level: 50
EVs: 244 HP / 116 Def / 140 SpD
Sassy Nature
- Heavy Slam
- Leech Seed
- Flamethrower / Wide Guard
- Protect

180+ Atk Incineroar Flare Blitz vs. 244 HP / 124 Def Celesteela: 168-200 (82.7 - 98.5%) -- guaranteed 2HKO
252 Atk Mega Rayquaza Dragon Ascent vs. 244 HP / 116 Def Celesteela: 57-67 (28 - 33%) -- guaranteed 4HKO
4 Atk Mega Rayquaza Dragon Ascent vs. 244 HP / 116 Def Celesteela: 49-58 (24.1 - 28.5%) -- 98.2% chance to 4HKO

Before Misty Seed
+2 252 SpA Fairy Aura Xerneas Moonblast vs. 244 HP / 140+ SpD Celesteela: 85-101 (41.8 - 49.7%) -- guaranteed 3HKO
252 SpA Fairy Aura Xerneas Moonblast vs. 244 HP / 140+ SpD Celesteela: 42-51 (20.6 - 25.1%) -- Miniscule chance to 4HKO
252 SpA Nihilego Continental Crush (160 BP) vs. 244 HP / 140+ SpD Celesteela: 106-126 (52.2 - 62%) -- guaranteed 2HKO
52+ SpA Primal Kyogre Water Spout (150 BP) vs. 244 HP / 140+ SpD Celesteela in Heavy Rain: 142-168 (69.9 - 82.7%) -- guaranteed 2HKO

After Misty Seed
252 SpA Fairy Aura Xerneas Moonblast vs. +1 244 HP / 140+ SpD Celesteela: 29-34 (14.2 - 16.7%) -- possible 6HKO
+2 252 SpA Fairy Aura Xerneas Moonblast vs. +1 244 HP / 140+ SpD Celesteela: 57-67 (28 - 33%) -- guaranteed 4HKO
252 SpA Nihilego Continental Crush (160 BP) vs. +1 244 HP / 140+ SpD Celesteela: 72-85 (35.4 - 41.8%) -- guaranteed 3HKO
52+ SpA Primal Kyogre Water Spout (150 BP) vs. +1 244 HP / 140+ SpD Celesteela in Heavy Rain: 94-112 (46.3 - 55.1%) -- 68% chance to 2HKO