As I suggest before about make it tradeable…
Now I come up with another idea. Why not make it a quest by Job master NPC instead?
Since many people not satisfied at all when they got card from field boss. Maybe make Necromancer and Sorcerer master NPC give a limit xx per day repeat quest. Quest make us kill specific field boss and we will get that specific boss card after report to master NPC with out need to get one from random loot cube.
Please note that normal quest boss will drop card normally as they usually do. So class that need card to advance can still do it. (Unless it need card from field boss? I have no clue what card bokor advance quest need).