Updated solution from Philmnator at Oct 3
The game has update and it seems the glitch resolved, anyone still not unlock the game please try update and open the game the achievement should unlock immediately. Update solution from several gamers at Sep 17
Reported from several gamers it seems clear all Level 1 - Level 5 cards will unlock this achievement, if you still stuck it won't unlock, you may try to clear all cards from Level 1 to 5 and see if it popped.WARNING: INITIAL "FREE" SOLUTION NO LONGER WORK SINCE 8 SEP
While the screen show unlock all is free, when click and purchase it will ask for real money so below solution seems no longer work.
Also some users report even with lv 1-5 completed and unlock level 6, the achievement won't unlock.
As now seems the achievement glitched without paying real money. Several gamers simply play through from level 1 to level 5 and thus level 6 unlocked won't pop for this achievement.Below solution seems no longer work
As of today (7 Sep), you can click the shopping cart icon at top right and the first "Unlock all" should be free, simply "purchase" it and you will instantly unlock this achievement.