The official method is obviously out, since you'd need to run a transfer tool on the source console via download play
The by far most convenient (you can transfer as much as you want in one swoop, copy instead of moving, no gimmicks no bullshit) unofficial method is to backup the saves of both games and use PkHeX to drag&drop them in and out
I'm not aware of any online method, except possibly via an unofficial GTS allowing you to upload your pokemon (one at a time, even worse performance than the official method), automatically converting them to the newer format, and letting you receive them (also one at a time) on BW12