The Ironclad Counter:
Mega-Metagross <---(I hate Hula)
A E G I S L A S H
Aegislash @ Weakness Policy
Ability: Stance Change
EVs: 252 HP / 252 SpA / 4 SpD
Quiet Nature
IVs: 0 Spe
- King's Shield
- Shadow Ball
- Flash Cannon / Sacred Sword
- Shadow Sneak
Yeah this beats any variant of Nihilego. Resists/immunities to both STABs plus insane defensive stats while in shield form means that Nihilego is doing 0 to Aegislash. Even rare HP Fire variants do practically nothing, trigger WP, and bring about their own demise swifter. One could also run a more Sp.Def Aegislash with Leftovers and Sub over Shadow Sneak or Shadow Ball to counter Nihilego even harder, but that's not worth it imo.
Offensive Calcs:
252+ SpA Aegislash-Blade Flash Cannon vs. 4 HP / 0 SpD Nihilego: 134-158 (72.4 - 85.4%) -- guaranteed 2HKO
+2 252+ SpA Aegislash-Blade Flash Cannon vs. 4 HP / 0 SpD Nihilego: 266-314 (143.7 - 169.7%) -- guaranteed OHKO
0 Atk Aegislash-Blade Shadow Sneak vs. 4 HP / 0 Def Nihilego: 58-69 (31.3 - 37.2%) -- 76.7% chance to 3HKO
0 Atk Aegislash-Blade Sacred Sword vs. 4 HP / 0 Def Nihilego: 86-102 (46.4 - 55.1%) -- 64.5% chance to 2HKO
Flash Cannon into Sneak OHKOs standard Nihilego 100% of the time, and even uninvested Sacred Sword has a decent chance to 2HKO.
Defensive Calcs:
252 SpA Nihilego Power Gem vs. 252 HP / 4 SpD Aegislash-Shield: 24-28 (14.3 - 16.7%) -- possible 6HKO
252 SpA Nihilego Thunderbolt vs. 252 HP / 4 SpD Aegislash-Shield: 36-43 (21.5 - 25.7%) -- 1.1% chance to 4HKO
252 SpA Nihilego Hidden Power Fire vs. 252 HP / 4 SpD Aegislash-Shield: 48-58 (28.7 - 34.7%) -- 3.4% chance to 3HKO
Even if you were Taunted into Blade form and couldn't KS, Nihilego still can't OHKO with anything and you get the FC kill after WP activates.
252 SpA Nihilego Hidden Power Fire vs. 252 HP / 4 SpD Aegislash-Blade: 114-136 (68.2 - 81.4%) -- guaranteed 2HKO
GG
Tapu Lillie Nihilego
252 SpA Nihilego Hidden Power Fire vs. 0 HP / 0 SpD Metagross-Mega: 64-76 (41.2 - 49%) -- guaranteed 3HKO