the chance now is.... 1/127*15%.
1 on 127 miths * 15 on goldboxes / 5 cards= 4.23% if you buy one gold box.
1 on 127 / 1 card (on mith box) = 1.27% if you buy one mith box.
1 on 127 / 3 cards (on big myth box) = 4.23% if you buy one big box.
so, if you buy them on 2x hotsale, those % are doubled...
and of course you can get it from normal boxes too:
-1% to get on normal boxes 1 random myth, give you = 0.04233333% to get it for free.
if dont like maths, the quick answer is: you need loot of lucky.