Moves
Thunderbolt provides coverage against troublesome bulky Water-types such as Empoleon, Slowbro, and Tentacruel. With Electrium Z, it turns into a powerful Gigavolt Havoc capable of OHKOing the aforementioned Water-types after some chip damage while heavily damaging Steel-types like Scizor, Bisharp, and Cobalion. Grass Knot nails various Ground-types that could otherwise wall Nihilego, like Krookodile, Hippowdon, and Swampert. Hidden Power Ice allows Nihilego to keep entry hazards up against Gligar and is also its strongest attack to punish Nidoqueen and Nidoking, which otherwise sponge anything Nihilego can throw at them and can absorb its Toxic Spikes. Power Gem is an option over Grass Knot, sacrificing coverage on Ground-types for a STAB attack capable of hitting Fire-types and Ice-types like Rotom-H, Kyurem, and Chandelure super effectively while hitting Scizor harder than Thunderbolt. It can also hit Flying-types harder than Thunderbolt, KOing Moltres and Crobat in one hit. Nihilego can set Stealth Rock as it forces switches, punishing the opponent's switches with chip damage. Protect can be used in tandem with Black Sludge to scout for moves on Choice-locked Pokemon, like Choice Band Scizor and Terrakion and Choice Scarf Krookodile, while passively restoring Nihilego's HP. Toxic Spikes can also be set on forced switches, racking up poison damage on foes as they come in. Thunder can be run in place of Thunderbolt to more easily power through Gigavolt Havoc's targets, requiring notably less chip damage to OHKO Empoleon and Tentacruel while always OHKOing Bisharp after Stealth Rock. However, it has limited use outside of this due to its shaky accuracy despite its higher power.
Set Details
Black Sludge should be run as Nihilego's item to improve its longevity and when used with Protect it can passively restore its HP as it scouts foes' attacks. Electrium Z lets Nihilego lure and KO several Water- and Steel-types, such as Tentacruel, Empoleon, Doublade, and Bisharp, punishing them with heavy damage. Life Orb is another option if another teammate is running a Z-Move, putting power behind Nihilego's moves and notably allowing it to OHKO Primarina with Sludge Wave and 2HKO Empoleon with Thunderbolt. It can complicate Nihilego's entry hazard setting capabilities by cutting into its longevity, however. Beast Boost allows Nihilego to increase its Special Attack upon netting KOs. An EV spread of 80 HP / 176 SpA / 252 Spe allows Nihilego to boost its Speed instead of its Special Attack with Beast Boost, though this can notably cut into its damage output at times, preventing it from reliably OHKOing Swampert with Grass Knot, for instance.
Usage Tips
If Nihilego is running Stealth Rock or Toxic Spikes, it should attempt to set them early-game in order to immediately apply pressure to the opposing team. It should do this against special attackers that can't hit it hard, like Moltres, Mega Manectric, and Rotom-H, or against passive defensive Pokemon like Alomomola and Florges. After setting entry hazards, Nihilego should capitalize on its good coverage and power to try punching holes in opposing teams. Later on in the game, when its checks have been removed or weakened, Nihilego can try cleaning up thanks to its decent Speed and Beast Boost, especially if it is running the investment necessary to receive a Speed boost. If Protect is being run in the fourth slot, it should be used against potential Choice item Pokemon like Scizor, Krookodile, and Hydreigon to scout for their attack. This also allows Nihilego to passively restore health with Black Sludge to better handle Pokemon like Togekiss and Mega Manectric. If Toxic Spikes is being used, Nihilego should only set one layer against offensive teams, as most Pokemon on these teams won't stay in play long, making consistent poison damage an easy method of quickly chipping them down. Against defensive teams, however, two layers should be set in order to rack up poison damage over time against the passive Pokemon often found on these builds. Electrium Z is best activated against offensive Steel-types and bulky Water-types, such as Bisharp, Mega Aggron, and Mega Slowbro, as otherwise these Pokemon would be able to tank a hit and pressure Nihilego. Prediction is important in securing KOs against Pokemon that could tank a hit and KO Nihilego. For instance, if a Ground-type like Hippowdon or Krookodile is expected to switch in, Nihilego should try nabbing them with Grass Knot on the switch in order to secure the 2HKO without taking any damage itself. Nihilego shouldn't stay in on physical attackers that it can't outrun or KO, such as Mega Aerodactyl, Mega Steelix, and Mega Aggron, due to its awful physical bulk. It should also look out for powerful priority attacks from foes like Scizor, Bisharp, and Crawdaunt, Choice Scarf users like Infernape and Krookodile, and Pursuit from the aforementioned Scizor, Bisharp, and Krookodile.
Team Options
Nihilego fits well onto several different archetypes, ranging from hyper offense to balance, thanks to the useful offensive and defensive utility it provides to teams it's placed on. If Nihilego is running Stealth Rock or Toxic Spikes, setup sweepers like Latias, Mega Altaria, and Lucario appreciate its ability to chip down opposing teams. The latter two appreciate its ability to deal with Moltres as well. Wallbreakers like Mamoswine, Crawdaunt, and Volcanion also enjoy its entry hazard support for this reason. If Nihilego wishes to forego entry hazards for Protect, other setters like Mega Aggron, Cobalion, and Hippowdon, which can additionally support Nihilego with Sand Stream, can let it more comfortably do so while also allowing Nihilego to more easily muscle past opposing Pokemon. Steel-type checks like Infernape, Suicune, and Moltres can weaken or entirely remove a few particularly troublesome answers to Nihilego, such as Mega Aggron, Mega Steelix, and Klefki. Ground-type answers, such as Tsareena, Celebi, and Primarina, can also be important for easing Nihilego's matchups against them while also potentially allowing it to more comfortably run Power Gem. Pivots like Scizor and Hydreigon can bring Nihilego in more easily, since it at times won't be able to comfortably come into play on account of its poor Defense. Pokemon capable of tanking a few physical attacks, like Gligar, Doublade, and Cobalion, can help Nihilego more easily circumvent its physical frailty. In return, Nihilego can reliably come in on a few troublesome special attackers for the aforementioned teammates to face, such as Mega Manectric and Celebi. Pokemon that appreciate a sturdy check to Fairy-types, such as Hydreigon, Latias, and Mega Altaria, benefit from Nihilego's support. Infernape particularly pairs up well with Nihilego, appreciating its ability to check Flying-types and threaten Fairy-types like Mega Altaria and Primarina, checking Steel-types for it in return, and benefiting immensely from its Toxic Spikes support if being used. Specially frail Pokemon can also benefit from Nihilego's ability to blanket check a few important special attackers, meaning the likes of Bisharp and Slowbro will often appreciate it as a teammate.