Tapu Lele @ Choice Scarf
EVs: 6 Def / 252 SpA / 252 Spe
Ability: Psychic Surge
Modest Nature
- Dazzling Gleam
- Psychic
- Energy Ball
- Hidden Power [Fire]
Psychic Terrain is quite useful for blocking a lot of priority moves from grounded Pokémon and boosting the power of Psychic/Zen Headbutt. Often lead with either Dazzling Gleam or Psychic, the other moves are just coverage for when that’s not as useful.  Shadow ball is a decent replacement for Hidden Power Fire if you already caught Lele and can’t acquire another one.  Choice Scarf Dazzling Gleam allows it to hit before a lot of other things and can help break Focus Sash and Air Balloon on both opposing Pokémon to help Metagross get a KO. Otherwise it can hit with Psychic for a fast and potentially devastating attack on a single target.  There will likely be times where you may need to swap Tapu Lele for Koko and back just for the effects of the terrain alone. 

Metagross @ Metagrossite
EVs: 6 HP / 252 Atk / 252 Spe
Ability: Clear Body
Jolly Nature
- Iron Head
- Zen Headbutt
- Thunder Punch
- Stomping Tantrum
Iron Head instead of Bullet Punch for extra damage and because of Psychic Terrain. Zen Headbutt does well in Psychic Terrain but the accuracy can be an issue. Thunder Punch and Stomping Tantrum have both been useful coverage moves as well as not hitting allies. There’s a few fast fire types that will really hurt Mega Metagross, like Darmanitan, Salazzle and Mega Houndoom. You should attempt to remove those quickly. 

Celesteela @ Life Orb
EVs: 252 Atk / 6 SpD / 252 Spe
Ability: Beast Boost
Adamant Nature
- Heavy Slam
- Seed Bomb
- Earthquake
- Stone Edge 
The famous Ventysteela set but with a Life Orb for freedom of move choice. Although still frequently just picking Heavy Slam anyway. Celesteela gets sent out when either Metagross or Tapu Lele faint, although there will be times that Tapu Koko should be sent out first. Fast fire types for example.  Tapu Koko's Protect allows for Earthquake to be selected without harming an ally.

Tapu Koko @ Electrium Z
EVs: 118 HP / 252 Atk / 140 Spe
Ability: Electric Surge
Adamant Nature
- Wild Charge
- Brave Bird
- U-turn
- Protect
The set from the anti Red battle team I use, just kinda threw this on the team anyway and so far it’s worked well enough.  Protect is useful for letting Celesteela use Earthquake as well as just stalling a turn when you know something is gonna try and destroy Tapu Koko. Wild Charge and Brave Birb as main attacks and U-Turn for when you need to switch but also damage something.  Gigavolt Havoc for when Wild Charge just isn’t going to enough.  Electric Terrain is useful for boosting Wild Charge as well as helping prevent sleep status for grounded Pokémon or simply taking away the Psychic Terrain if an enemy is taking advantage of it.