Alright, I'm assuming you are fighting a free rules match on USM. If so, here is what I suggest:
You can play Necrozma 2 ways. I highly recommend this first one:
Necrozma-Dawn-Wings/-Dusk-Mane; Item: Ultranecrozium Z
-Moongeist Beam/Earth Power
-Rock Polish
-Photon Geyser
-Dragon Pulse
I know you got your Items planned out, but I don't think Weakness Policy or Base Necrozma are good if you use Groudon or Kyogre. Moongeist Beam is if you choose to Fuse with Lunala. Rock Polish makes you fast enough when you go Ultra to out speed stuff like Deoxys-Speed, Mega Mewtwo Y, other Ultra Necrozma, and +1 Spe Rayquaza.
If you can't fuse or want to use Weakness Policy, at least use this set:
Necrozma; Item: Weakness Policy
-Rock Polish
-Photon Geyser
-Earth Power
-Thunderbolt
Earth Power is for Steel Types and Thunderbolt is for Steel/Flying or Dark/Flying Types. Rock polish is so you can Speed Up into a Sweep. :P
Primal Groudon
-Fire Punch
-Stealth Rock/Stone Edge
-Precipice Blades
-Thunder Punch/Rock Polish
Rock Polish and Stone Edge is if you want a Sweeping Pdon, Thunder Punch and Stealth Rock is if you want to create defensive pressure.
Primal Kyogre
-Thunder
-Ice Beam
-Origin Pulse
-Calm Mind/Rest/Thunder Wave
Thunder never misses in Heavy Rain and is Kyogre's best move against other Water Types. Calm Mind is for Being a devastating special wall, Rest is if you want some recovery, and Thunder Wave is if you want Kyogre or another Member out speed something or shutting down setup sweepers in general.
Timid Mewtwo-Mega
-Psystrike
-Recover/Calm Mind
-Thunderbolt
-Ice Beam
Recover if you want to live Longer and Calm Mind if you want to sweep. The notorious Bolt Beam combo is for coverage.
I'm changing an item again. :P
I don't think you should carry more than one mega to prevent an item from becoming a dead weight. :P
Modest Mewtwo; Item: Choice Scarf
-Psystrike
-Trick
-Shadow Ball
-Flamethrower/Focus Blast
With a Scarf, Modest Mewtwo should out speed and OHKO many Psychic and Ghost types you may encounter. Flamethrower is for accurate coverage; Focus Blast is for stronger, inaccurate coverage. Trick allows you to shut down a defensive Pokémon or a sweeper and hopefully give Mewtwo a Leftovers and a take-down in return.
I don't think Life Orb will be good for the Rayquaza set I suggest, so I change the Item once more. :P
Rayquaza@Flyinium Z/Roseli Berry/Yache Berry
-Dragon Ascent
-Dragon Claw/Outrage
-Earthquake
-Dragon Dance
The Z-Crystal is if you chose to use Weakness Policy on Necrozma. Rayquaza won't be able to Mega Evolve holding a Z-Crystal, but you already have a Mega Stone on Mewtwo, so if you did, you would have a dead weight item on your team. Roseli Berry is to survive a Moonblast from Geomancy Xerneas, and Yache Berry is to tank an Ice type attack. Use Outrage if you are confident you won't run into Fairy Types or you plan to KO all of them before you use Rayquaza. Earthquake Takes care of Steel types.
That is what I think. :P
Hope I Helped!