This is a moveset for the standard Necrozma. Not Dusk Mane, Dawn Wings, or Ultra Necrozma; just regular, vanilla Necrozma.
Necrozma @ Leftovers
EVs: 252 HP, 80 Def, 176 Sp. Def
• Stealth Rock
• Knock Off
• Photon Geyser
• Morning Sun/X-Scissor/Brick Break
This Necrozma is best suited as a lead Pokémon, setting up Stealth Rocks, Knocking Off the opponents item, and having STAB damage in Photon Geyser. Assuming your Necrozma's Attack IV is higher than its Special Attack IV, Photon Geyser will be physical. If you don't have USUM, Psycho Cut will fill Photon Geyser's role just fine. The last move is up to your preference. Morning Sun (or Moonlight) would be reliable healing, but with low PP. X-Scissor is for coverage and Brick Break is weaker coverage, but would cover the four Fairy/Steel-types that X-Scissor misses out on. Pick whichever one fits your teambuild/battling style best!
Vanilla Necrozma seems to be overshadowed by its new forms, but hopefully this moveset will bring it back into the limelight!