This worked for me. I put up an Icy Snow Vivillon for a Koffing. Traded a bunch of breeding reject Koffings for Xerneas, Yveltal, and Mewtwo, as well as the rest of the pokemon breedable I needed. Traded a Xerneas for a Unown, traded the Unown for a Virizion, and turned that into a dex entry for every single pokebank legend, excluding events.
Someone also traded a shiny level 100 6IV Landorus for something I put up, either Regigigas or Suicine, I forget. Got that cloned and traded the two of them for a 6IV ditto and a 5IV Shiny Hawlucha.
I did all that from one single Vivillon in two days. There's no excuse for just coming here and whinging you can't get pokebank exclusives. --- I have never made but one prayer to God, a very short one: "O Lord make my enemies ridiculous." And God granted it. - Voltaire