Wish would heal the user Pokemon only. You are allowed to bring out more than one Pokemon at once, but I would limit it to two. You can also switch the Pokemon since you want to use Wish. Wish doesn't affect the wild Pokemon, but you can heal it using a Potion.
Yeah, um, as it turns out Whiscash doesn't learn Sunny Day (unsurprisingly). I apologize, I don't know what I was thinking when I typed that. All you need to know is that because I screwed up things are gunna go a bit easier on you for a while