Well, first thing's first, I'll give you suggestions for challenges, but this is just stuff that works for me. If it doesn't make sense to you or something, feel free to try other methods! Anyway, I never use emulators for my challenges because I prefer the feel of a real game, plus I like to keep my teams. I also don't have an emulator on my Mac and I heard Mac emus don't let you hack, which is the only reason I would ever play on one in the first place honestly haha. But a lot of people prefer them, so it's understandable if you'd rather use that. Gym Challenge is a good place to start. :) Another thing people like to do here is merge challenges together. So, in this case, since you're going to be Erika for your gym leader challenge, if you're picking all Grass types, you could also sign up for the monotype challenge, and kill two birds with one stone. :D As long as you comply to the rules of both challenges, you can do that. And there isn't really an order you should try challenges. I mean, I just do whatever I feel like doing haha. In the future though, I do recommend trying a random challenge. :) They're my favorite, and totally fun. They'll have you use things you probably wouldn't have wanted to use yourself haha. And then there's Gender Discrimination which is only for HGSS, but a lot of people are having fun with it and it's something pretty different, so if you ever want to do something a little out of the norm, that one's up your ally, as well as the Time Warp challenge.
Also, we have a sort of chit chat thread called CI&D, where you can post challenge ideas, talk about what you're doing in your challenges, etc. So if you ever want more opinions than just mine, there are plenty of people there to help you out too. :)