The simple answer is you don't. Because of how restrictive rain is, what you need to do is overwhelm your enemy before they can retaliate. Of course, this is easier said than done. And if you think specs Kyurem rips rain apart, you haven't seen nevermeltice who runs roost and isn't as easily pressured
The only two common Pokemon that can reliably counter Kyurem, Scizor and Blissey, are not particularly easy to slot on a team. Scizor can fit on a rain team on the last slot but ultimately, this mon is next to worthless if the team isn't facing a Kyurem and Blissey has no business being on a rain team
At this point, you have two options, you either dump a lot of spdef into Ferrothorn or you deter Kyurem from clicking ice beam. The best choice for the second option is Volcanion, who quad resists ice beam and is neutral to freeze dry. Volcanion's presence should make Kyurem think to click freeze dry which would allow Ferrothorn to safely absorb and leech seed or heavily damage it
Thing is, the most common Kyurem set is nmi with roost, ice beam, freeze dry and earth power, which allows it to beat Heatran and chip away at Melmetal if it tries to come in. Specs, while still a thing, is slowly being passed over because of its heavy prediction reliance. If you are run into subroost Kyurem, then things are even easier for Ferrothorn. Just make sure you have iron head as your attacking stab instead of gyro ball. The same goes if you run into some random scarf sets
In the end, you do not counter Kyurem on a rain team. If you do, then you'll only end up making your rain team worse. The best way is simply to deter it. Yes, playing russian roulette against something as dangerous as Kyurem isn't ideal but the fact of the matter is rain teams just don't have that much options against it. This is true even last gen where Black Kyurem would just get a kill with its z move nearly every single time and it doesn't even have to use the z move since rain is mostly neutral to ice and those that resist it is naturally frail. Rain inherited these traits in gen eight so its either you play russian roulette or you make your rain team worse overall and most of the time, russian roulette is the chosen option as while Kyurem is common, it isn't common enough where it is on every other team unlike a certain flying lion
