Bug types are great in this battle. Pinsir, Heracross, Vivillon and Araquanid are good picks. Pinsir and Heracross just hit so damn hard, Araquanid can abuse the Trick Room with Water Bubble boosted attacks, and reduce the opposing pokemon attack stat with Lunge. I found Vivillon sort of helpful, thanks to the Sleep Powder support, alongside good STAB in Hurricane (mainly for Gallade) and Bug Buzz. You can get Heracross, Pinsir and Araquanid in the Route 1, and Vivillon in the Jasper Ward. Strategy wise, you should lead off with Decidueye ans Sharpedo to take out the Gallade and the Reuniclus. You might want to use some type gems, though, since Radomus has many bulky options. You can use Sharpedo to nab a KO on Metagross with Destiny Bond, and by using a bug type, Malamar gets molded by being 4x weak to bug types. Gardevoir dies to Klinklang, if it outspeeds you, though, you can make Slowking use Trick Room, or just make Gardevoir attack a different pokemon