Replace earthquake with Precipice Blades on Groudon. Psyshock is good on Xerneas, as it counters its weakness to poison types. You need to put Dragon Dance, Double Edge, and Earthquake on Salamence, as they are moves crucial to its battling strategy. Give Necrozma Dusk-Mane Solgalium Z or Ultranecrozium Z. Also, if you put Ultranecrozium Z on Necrozma Dusk-mane, you might want to replace Thunder Wave with Outrage for STAB when using Ultra Burst. Otherwise, relace Thunder Wave with Earthquake. If you can get a TM for it, you can put Poison Jab on Marshadow, to counter against fairy types. At level 56, Marshadow learns sucker punch, which is an excellent counter against Mega Alakazam and other fast-moving psychic Pokemon. You might also want to give Marshadow Ghostium Z (Base Move: Spectral Thief), Muscle Band, or Life Orb, as Choice Band can be annoying if you're stuck using an ineffective move. Groudon learns Solar Beam at level 65, which could be used as a counter against Water and Ground, but, if you want an all-physical moveset, or if your Groudon has an Adamant or other special attack-lowering nature, you probably shouldn't teach it. Naganadel is ok, but it is optional to replace fire blast with Flamethrower, as Fire Blast is unreliable in accuracy. Here are the changes I'm suggesting:
Remnant (Naganadel) @ Dragonium Z
Ability: Beast Boost
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
IVs: 0 Atk
- Draco Meteor
- Sludge Wave
- Nasty Plot
- Flamethrower
The Don (Groudon-Primal) @ Red Orb
Ability: Desolate Land
Shiny: Yes
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- Stealth Rock
- Rock Polish
- Precipice Blades
- Stone Edge/Solar Beam
Marshadow @ Choice Band/Muscle Band/Ghostium Z/Life Orb
Ability: Technician
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- Sucker Punch
- Spectral Thief
- Drain Punch
- Poison Jab
Boogies (Salamence-Mega) (F) @ Salamencite
Ability: Intimidate
EVs: 252 HP / 204 SpD / 52 Spe
Careful Nature
- Earthquake
- Double-Edge
- Roost
- Dragon Dance
Necrozma-Dusk-Mane @ Solgalium Z/Ultranecrozium Z
Ability: Prism Armor
EVs: 252 HP / 252 Def / 4 SpD
Impish Nature
- Sunsteel Strike
- Morning Sun
- Photon Geyser
- Earthquake/Outrage
