I’ll say this based on my own experiences so far.
If one just invested in rayquaza and only lucky traded ray rays, then it wouldn’t be that hard to make several of them for such challanges. I myself do mostly the daily raid and here and there a few extra ones, since I am not planning on spending money on the game outside of sometimes buying a box with a good offer. I in general don’t really struggle with rare candy or stardust that much, but it’s hard to get mons which I consider IV wise good enough to max. In raids escpeically I have really bad RNG regarding good IVs, considering I’m slowly approaching 1000 raids and from all of those I only got one mon with perfect IVs. Compared to eggs, research tasks, rocket battles, etc I’m really having bad luck here. While I haven’t gotten a perfect IV mon yet with a luck friend, I already got several 98% that way and that would be my. With said feature one was way more chances to snach something good compared to raids and honestly if only traded rayquaza so far and only put rare candy into this species, then I would likely have 2 teams so far as well. I do have however my reasons why I didn’t do this.
The problem is that one would really need to aim just for these specific challanges and find someone else who is willing to do the same thing. The latter part is probably the hardest. If that doesn’t happen, then there really is no point in just focusing on the very best species and not giving the cheaper and nearly just as good alternatives a try. I myself like to have varied teams, so despite giving the advantage to the top performer of a certain type, I do still power up the alternative picks.
With Rayquaza the problem is also that it kind of lacks flexibility. It’s currently DPS wise the best dragon in raids, but elsewhere other dragons beat it. In rocket battles and pvp it fails to impress compared to the other dragons, since Dialga, Dragonite and Latios are way better there, due to having a better fast move, acess to better charge moves and also have more bulk. Despite all the dragons that we have I would still say that Dragonite is overall the best pick for new players that lack the other dragons, since it’s always avaliable, it’s best moves aren’t exclusive and while not being the top pick, it’s still overall the best when it comes to raids, pvp and rocket battles, due to being very good in all of them.
Rayquaza does however have the advantage of being quite future proof. It already has the best moveset in game and its singature move is of the flying type, which even if it gets implemented and is very powerful won’t be that important, due to being rarely needed. Although bonus points on being useful from it’s secondary typing, which one can’t say of dragonite, salamance, palkia…
This being said, black Kyurem is coming sooner or later in PoGO and it will be noticably better than the other dragons DPS wise, unless Niantic decides to make its movepool worse before release. When that day will come it should end up being one of the top picks for rare candy investments and for trades with lucky friends. Poor ray.