So, Necrozma's one of the best wincons in the alpha by a large margin, thanks to stupid bulk, good enough speed, and incredibly high spA. It however has a few problems, chiefly among them, its vulnerability to dark types, and the wonderful status move toxic. While it really can't ever break past Dark types, you can easily solve the toxic issue using one of these two sets:
Necrozma @ Leftovers
Ability: Prism Armor
EVs: 248 HP / 152 Def / 108 Spe
Bold Nature
IVs: 0 Atk
- Substitute
- Psyshock
- Moonlight
- Calm Mind
Basically, Sub-Calm Mind lets you dodge toxic, and thanks to high enough speed, you can set up on like half the tier. Additionally, steelix struggles to efficiently break your subs (136+ Atk Steelix Earthquake vs. 252 HP / 152+ Def Necrozma: 94-112 (23.6 - 28.1%)), and substitute means you can freely set up in Hitmontop, instead of hoping they don't get ballsy and click toxic.
There is however another way to dodge toxic, and this set is arguably even more threatening in a late game situation.
Necrozma @ Leftovers
Ability: Prism Armor
EVs: 252 HP / 252 Def / 4 SpD
Bold Nature
IVs: 0 Atk
- Rest
- Sleep Talk
- Calm Mind
- Stored Power / Psyshock
Similar to Cro-Cune, you basically aim to survive until the late game and then just win against any team that can't 3hko you on the physical side. Fairly simple set, and very effective, just make sure you can take on Yanmega.
Your best partners include Garbodor, to trivially wear down dark types such as Sneasel and Pangoro, as well as set up spikes to punish the switches Necro causes. Steel types such as Steelix or Silvally-Steel can also be very helpful for taking on Ribombee, and pretending they win vs Yanmega, while either setting up Stealth Rock or providing Toxic/Parting Shot support, while Necro loves to set up in the fighting types that threaten them. Fairy types love to switch into the darks that Necrozma can lure in, and beat Sableye, which is always a help. Strong fighting types can easily take advantage of Necro luring in stuff like Sneasel and Steelix. Finally, Porygon-2 is basically the best tank ever, and appreciates Necrozma setting up on the fighting types that threaten it.
Can't take credit for these sets (if you did, pm me, and I can give you credit), but they're rather effective.
As for ways to beat it, Roar Steelix can win in a non-last mon situation, Yanmega and Ribombee can threaten it out with SE bug moves, and Dark types basically nullify it, as it can rarely hit them. Toxic spikes is also a potent way to deal with the mon, as while its easy enough to carry a heal bell mon, hazard removal remains super hard in the current meta. Haze is also a good way to punish it, as setting it back to +0 means its hard to set up again, especially with how limited moonlight's PP is.