Is it just Ludicolo as a rain abuser? If so, you could try out Gigalith. Spdef always switches into it if you're healthy (takes around 50 from pump in sand) and is able to remove the rain. Encore Whimsicott sounds pretty good against it. You're able to encore them into whatever move they click on you (most likely Ice Beam) and react accordingly. Druddigon might be what you're looking for as well, trading damage on drudd for a Glare on Ludi sounds worth it if otherwise your team gets destroyed.
If it's rain as a whole I might need to see more of the team but, generally having one or two of those options I mentioned alongside maybe some priority or spikes, and making key trades in battle should do the job. Keep up the offensive pressure when needed to make sure they cant get up rain every time without being punished.
If any of these don't work out feel free to pm me on here or discord (on my profile) and we can talk more