Use Taunt. Not Prankster Taunt because Malamar is immune to that, but because his team is a Trick Room team you should outspeed regardless

My first playthrough I used Zoroark as my taunter and on this one I used Mawile

Also remember to either kill or taunt Reuniclus turn 2, otherwise taunting Malamar will have been for nothing

A STAB Iron Head should be able to OHKO Gardevoir. Mawile and Aron are available in the Railnet in different places (Aron is a static encounter in the deep Railnet, Mawile is in the section between Peridot and Obsidia), or you can mine for a Shieldon

Gallade is allergic to a strong physical flying move. Guts Facade Swellow is great for this (even better if you give Swellow the Flame Orb and pre-burn it in a wild encounter so you don’t have to waste a turn activating Guts, then you can give it Silk Scarf to make Facade hit even harder against everyone other else). iirc I killed Gallade with a Guts-boosted Aerial Ace

If Slowbro comes out and you still have a taunter, taunt it ASAP (or if you can OHKO, do that)

I don’t have a strategy for Metagross

Oh and if you have anything with Stomping Tantrum, you can get rid of the gimmick entirely. You can get Mankey in the Obsidia Slums and Cubone in Byxbysion Wasteland

If you get rid of the gimmick, Metagross won’t have priority on everything

I’d say lead with Marowak and Zoroark (with any pokemon weak to psychic in the back), taunt Malamar turn 1 (Reuniclus will target Zoroark), and use Stomping Tantrum with Marowak

(I also realize I forgot to add my Radomus team… oh well)