Event Pokémon can theoretically only be traded directly via link trade, the GTS is off limits. In Generation 6, there were still scores of mythicals in the GTS, especially Jirachi for some reason, but this generation has tightened its security with regard to Pokémon obtained through illegitimate methods, and it seems to have scared off legit mythicals as well. Hacked Pokémon are less likely to come with a Cherish Ribbon or similar, meaning that they could still be traded via the GTS. If you missed the event last year and desperately want her, you'll need to trade directly. I have 3 if you can make me a better offer than a Typhlosion.
Work out what you have, or are capable of acquiring, because there is essentially a hierarchy when trading Pokémon. Marshadow is probably sitting at the top right now since it's still off limits in some countries, the next most prized would probably be Meloetta, Genesect and Keldeo. The cheaper mythicals other than Deoxys would include Mew, Celebi and Jirachi (who was freakishly easy to obtain a couple of years ago). Sometimes a certain shiny or item might win you a good deal. You just have to keep trading up or get ridiculously lucky.