Nihilego is a great Pokemon with its tricky speed, just outspeeding Garchomp and the base 100s. It also removes Toxic Spikes and has a unique typing. Its physical side is a shame but its special side makes up for it. I recommend you to avoid faster physical Pokemon and especially ground types. Despite having some flaws, it works great as a lead and sweeper. My moveset would be:
[email protected] Sash/Poisonium Z
252 SpA/252 Spe/4 Def
-Stealth Rock/Toxic Spikes/Thunderbolt/Psyshock
If you choose a hazard as a move, I recommend giving it a Focus Sash. Otherwise, give it a Poisonium Z so that it can ensure a kill against a non-poison-resisting Pokemon and get a Beast Boost in SpA. From now on, continue sweeping. Sludge Wave and Power Gem are STAB. Grass Knot is for Ground types which resist both STAB moves. Thunderbolt is for Toxapex and Psyshock is to nail Chansey as well as Toxapex.