Just go after the highest raid boss that you can easily beat every hour - maybe a few below your max (but over level 20 or 25). The more of them you beat, the better chances at getting the card. It is simply a matter of probability and diligence. You won't just get one, you'll get two or three.
Don't keep trying to beat the next highest level boss. I have found that does not work. Every once in a while, test your skills and try to move up, but otherwise, just keep smashing them.