Yeah it's a really fun challenge and I definately recommend it! However it's only possible during Guzzlord, Kartana and Pheromosa/Buzzwole and you have to be prepared with the top counters against each of them. I'd recommend doing it during Pheromosa/Buzzwole Raid Hour this Friday as Kartana is the only other one that can be done solo and It is rather difficult without weather boost, even with the best fire type attackers. But for Pheromosa and Buzzwole you should only need one Mega Rayquaza that you can keep healing each time it faints to beat them, even without weather boost. Anyways yeah I definitely recommend giving this challenge a try and good luck with it if you do!

That sucks, it is featured as the reward for I believe the Win 3 Raids task of the free Inbound from Ultra Space timed research so you should be able to get one through that if you still need one, But It will most likely be back in Raids sometime soon anyway potentially with the Halloween event this October👍