With any character who has strong zoning and who plays at a high level, you have to be patient with Rain. Dash blocking and advancing inwards while trying to make the best reads you can; like Bodam said, EX teleport can be useful also. You have to be careful to not just throw it out too much though, because it is punishable.
Lightning is something you also want to use to keep your opponent in check, but don't throw it out if you're too close to them. If it hits, react immediately by throwing a waterball and dragging their asses towards you. You can now either opt to go for pressure by letting them drop in front of you, go for a mixup or you can combo off of it with 4 RH while they're in the air. Lightning has really good startup frames and can stuff alot of stuff, you just have to make decent reads with it.
Really when it comes down to it, it's all patience when playing against characters that zone. Ermac to be honest I think is a 6-4 in Rain's favor though because he doesn't Zone more than he does space and once you're in on him, he has a really hard time getting Rain off of him. I mean there's really a myriad of things you can do. You can try to catch them with a standing or crouching jab if they're close; if they jump back it will catch them and you can then do a 43 into waterball or lightning; All in all get in close to them the best you can without losing too much in the mean time.
If they come at you go toe to toe with them; even Cage doesn't want any of Rain's offense. If they become aggressive don't be afraid to abuse armored dash or a GKC and bust through their offense with your own.
It's not as coherent of advice as I'd like to give but I really believe I'm a better player than I am a teacher unfortunately. Alot of this is just stuff I've got a feel for after maining Rain since the hour of his release.