Keep plugging away knowing they'll turn up eventually, or just give in and buy the related item. It's pot luck whether or not the hard/crushing treasure turns up. I've received barely any of them, I keep getting the easy/normal ones, but I'm not that fussed for the unlockables anyway.
If you are talking about treasures related to skins, well, then, get used to it, cause that's how it is. You must farm EXTENSIVELY for skins and it takes a really, really, long time.
The other option is to just outright buy the skin in the store as they have been made available recently.