well you will have to wait for the event because thats the only legit way.
or you could hack or use an action replay but thats cheating.
my suggestion is be patient because it hasent came to japan yet so it wont be here for at least a year
There's an event for it coming out soon, you can see it in japanese on youtube.
this is the video I suggest:
It's pretty cool so check it out. The move it learns is Secret Sword, only for Keldeo to learn. It's a fighting type move, 100 accuracy and 85 power. It's pretty cool, but you need the three legendary dudes, Cobalion, Virizion and Terrakion in order to get it to learn the move.