I like what sm said, I love to Toxic stall AI XD
Anyway, I have a few suggestions that involve an offensive strategy.
Pray that one of your party members has HP Ice / Dragon
Fairly simple. Go to PWT and for 18 BP. The HP checker is... at a booth outside of the arena at PWT apparently. Nice. Anyway, HP Ice will fair well against all of Drayden's team, especially Flygon with HP Ice.
Welp I read your comment oof. The only thing I can really say is maybe try re-catching some Pokemon? It helps with my next suggestion too.
EV train your Pokemon (to an extent)
You know, it took me forever to think of this. You see, this suggestion started as "Teach Serperior Dragon Tail" until I realized that Dragon Tail is received after beating Drayden. Then I said "Breed Disable Onto Cacnea" yeah you can't breed until after the Champion. Then I was getting a Pokemon with Hail for Weather Ball, but uh you would hafta cheat the mono-grass challenge.
That being said, now we have come to this. Hopefully your EVs aren't maxed out or you're going to need some EV resetting Berries. And those are annoying to get.
Anyway, I suggest, if you do EV train your Pokemon, to EV train them in the stats that will compliment their moves and Defense / HP to better tank hits from Drayden's Pokemon (though you probably want to plan ahead as well, for the Elite 4 and Champion). I wouldn't advise maxing out EVs, that would be quite a lot of work, but it would definitely help to EV train in specific stats.
This isn't really a full suggestion, to be honest if you were to grind and overlevel you should do it alongside EV training since the most efficient method in my opinion is knocking out Pokemon anyway, plus it wouldn't hurt to have HP Ice / Dragon on an overlevelled Pokemon. This is what everyone is going to suggest anyway, I'm simply stating it because overlevelling is the most basic thing you can do to beat any in-game trainer that is hard for a team to beat. Again, you can kill two birds with one stone by grinding while EV training.
That's about it, good luck, and I hope this helps! :)