Ah, Radomus is a perfect example of where you need to prepare. Yes, Poison is weak to Psychic. However, there are a few Poison/Dark types out there, handily nullifying the weakness. In fact, the Poison-typing also covers for Dark's weakness to Fighting, which Radomus has a few of (Superpower on Contrary Malamar and Close Combat on Gallade). Furthermore, keep a Nido in reserve for his Metagross.
Also take advantage of the fact that, despite Radomus' team being built to abuse Trick Room (Which is to say, everyone's super slow), he won't actually use it in most cases. Take advantage of that.
This is just one specific example. There are plenty of strategies you can use, and you'll need to use for the various leaders you'll face.