1st - I traded my xerneus for a ratata. 2nd - Breeded a box of ratata. 3rd - Placed ratatas in GTS asking for xerneus/yvetal each. 4th - Within 5mins~1hour, each trade accomplished. 5th - Gotten full box of xerneus/yvetals. 6th - Traded all of them for past gen legendaries. 7th - Profit~ Gotten a box of pokebank legends.
Trading shinies. So far I've gotten both lati's, thundurus, landorus, heatran and maybe others just by trading random friend safari shinies that have bad natures. People eat them up on GTS. traded a shiny skiddo for a suicune. The skiddo wasn't mine but I imagine it was obtained through Pokeradar, so if you are good at getting shinies through that you should be able to get all tradable bank legends quickly.
If you can get even one pokebank legend, you should be able to get all of the ones that are tradable on GTS, except for maybe box legends unless you're lucky.