Radomus is a pain in the ass since the gym's field effect means the move Psychic gets an additional Rock type damage. That means that Crobat and Infernape of yours are especially in danger. They won't last long in the fight, but they'll be useful in the beginning if they can hold out.
He'll open with Exeggutor and Reuniclus. Focus on Exeggutor since Reuniclus is relatively "safe." Open with Magnezone, and Crobat (try saving Infernape for Metagross) to OHKO Exeggutor--and I do mean OHKO because Crobat, with its weaknesses to both Psychic and Rock, means it'll be OHKO'd guaranteed. If it knows Acrobatics, get rid of any item it's holding and hope for the best. If Crobat can't OHKO it, hopefully Magnezone can finish it off. After taking down Execcutor, Radomus will usually switch in either G.Gardevoir or *shudder* Malamar.
Keep training your Magnezone since it'll be a good tank for the battle, and a key player in getting rid of G.Gardevoir. Its Steel typing guards against Psychic, Rock, and Fairy, and its defense-oriented stats can take a hit from Dark- and Ghost-type attacks. It should know Flash Cannon, and hopefully, with backup, it'll take down G.Gardevoir in 2-3 hits.
However, if he brings in Malamar instead of G.Gardevoir, you're out of luck with Magnezone since there's some glitch that means Malamar knows Focus Blast. Seriously. Out of all of Radomus's team, it was that goddamn Focus Blast Malamar that determined whether I won or lost. Malamar is fairly meaty as well and its type combo means it's only weak to Bug and Fairy. This is a Pokemon you'll just have to brute force. I suggest using Gyarados for its Dragon Dance and, if it still knows the move, Bite.
For me, those were the true hurdles. Your Zoroarks and Gyarados (if it lasts) should be able to handle things okay. If Magnezone's still around, you can keep using it as a tank to keep your Zoroarks healthy. An Electric-type would be useful in taking down his Slowking, but the Zoroarks or Gyarados should be able to deal with it. Your Infernape might be able to take down the Metagross.