I've had a good bit of success against Malamar with EspGarb. Evolving Eevee straight into Espeon T1 and taking a KO on an Active Inkay is awesome, and automatically sets the opponent back a Malamar. They generally can't fire off an attack that could call for a retaliatory KO, so you save yourself a turn. At this point, you can start setting up Garbotoxin and pinning down other Malamar or Inkay via Guzma if you fear the Field Blower.
If you're playing against the Psychic variant, targeting the Malamar is more important than going for the Necrozma or Dawn Wings Necrozma because of the need to have three Energy to attack. OG Necrozma-GX isn't something you have to really worry about because Espeon and Trashalanche hit for Weakness, but Dawn Wings could be a little more problematic. I'd switch my approach to Just make sure they're not fueling up those big guys via Malamar and it'll be more helpful in the long run. (Whether you do this by Garbotoxin or Guzma-ing them up, you'll be fine.)
If you're playing against the Ultra variant, I think the matchup is even better. Lockdown with Garbotoxin, Ultra can't hit you for Weakness and still needs a considerable amount of Energy to attack, and that version is a little less consistent with the need for Metal Energy. Ultra has 190 HP, however, so I'd Guzma those out, Confuse them with Psybeam, and bring those down to 160 so Garbodor hits those numbers without the need for Choice Band. Between Max Elixir, Mysterious Treasure, and Ultra Ball, they should be playing enough Items for damage output to be a nonissue.
I have absolutely no experience with BeastBox so that's all I've got for you.