I'd be cool with maybe a "list" where the hardest challenge is given, as they do with suggestions (unless it's the only location to get an item) with a handful of ? for however many other mobs there are to get it. If you don't want to do the footwork to explore you can go for the hard one, but if you're willing to put in the time, you can find the others, and you still have an idea for what gives it. Or they could do silhouettes of the mob and for each one you kill as a BLU, even if you don't get the skill, it gets filled in with an image so you know you're on the right track just keep trying.