M-3 Singles final 34th place (rating 2513). The concept was to "push as hard as possible with Gyarados, which works well in the meta," starting from the judgment that Gyarados is advantageous against most top-tier meta Pokémon, excluding Meowscarada and Glimmora (screens).
Gyarados: Adamant, A-max, semi-fast to outspeed mirror matches. The main star of a build used for over 600 wins. The core concept is to counter Sticky Web when the opponent relies on Meowscarada or Choice Scarf Garchomp for Gyarados countermeasures.
Pinsir (Mega): A-max, targets 102 Speed Pokémon. The second ace for Blaziken and Lopunny. Its ability to use Flying-type STAB moves with priority is a strong point.
Gholdengo: C/S-max, remaining points in HB. A sub-attacker with good type synergy with Gyarados and Pinsir. With a Shuca Berry, it can surprise Garchomp, Hippowdon, and Florges, and with Sticky Web, it can even outspeed Choice Scarf Baxcalibur. It also has Thunderbolt for a surprise attack against opposing Gyarados.
Hipowdon: D-max. An overwhelming entry hazard setter that can forcibly create an advantage even in unfavorable matchups with Yawn.
Araquanid: A-max to one-shot Mega Raichu Y, max Speed. Initially, Slurpuff was used, but Mega Raichu's Substitute usage was too high, so it was switched to Araquanid to endure with Focus Sash and counterattack.
Grimmsnarl: Fairy-type for Hydreigon. Loose EVs (roughly faster than Primarina, maximizing bulk and chip damage).
The basic lead is Hippowdon or Araquanid + Gholdengo + Mega (whichever of Gyarados/Pinsir is advantageous). If the Mega uses a stat-boosting move in front of Primarina, it will be Encore-locked, so other Pokémon should chip away first. When the situation is so favorable that everything can be entrusted to the Mega, a lead including Grimmsnarl is also used.
Mega Raichu Y (Sticky Web is essential, immediately dangerous if blocked by Substitute), rain teams, Focus Sash Meowscarada (no countermeasure), Klefki (not fatal but not advantageous either).