Psychic toolbox build pairing Mega Altaria ex's Mega Harmony bench scaling with Greninja's Water Shuriken chip damage and Giratina's free-retreat utility.

Mega Altaria ex
Swablu
Giratina
Greninja
Froakie
Meloetta
Copycat
Professor’s Research
Cyrus
Rare Candy
Poké Ball
Protective Poncho
Giant Cape
Source decklists referenced for this guide:
Open Swablu and Froakie, bench Giratina and Meloetta and attach Psychic energy. Chain Professor's Research / Copycat / Cyrus to dig for Rare Candy and evolve into Greninja for free Water Shuriken pings. Mega Evolve into Mega Altaria ex with a wide bench to swing Mega Harmony for 130+ damage. Detailed matchup data will be updated as the format develops.
Main attacker — Mega Harmony deals 40 plus 30 per benched Pokémon.
Free chip — Water Shuriken pings 20 damage to any opposing Pokémon each turn.
Pivot — Levitate gives it free retreat with any energy attached.
Disruption — Strange Singing pulls a random opposing Pokémon active.
Pulls a damaged opposing Pokémon active for a closing knockout.
Bench Swablu, Froakie, Giratina and Meloetta, attach Psychic energy and chain draw Supporters.
Rare Candy into Greninja for Water Shuriken pings while Mega Evolving into Mega Altaria ex with a wide bench.
Close with Mega Harmony 130+ swings backed by Cyrus pulls and Meloetta disruption.