Figuring out that you have to kill Rex second, and then pulling it off.
Unfortunately it was
rather obscure and easily missed. There was nothing wrong with the challenge itself, but it wasn't obvious what you needed to do here.
EDIT: I'm having a lot of trouble actually distinguishing between the ordinary background and the highlighting for a "selected" prep.
EDIT2: If you've already cleared the game such that all trophies have exactly the same price from the taxidermist, the "sell for 25 gold" option should be automatic on earlier boss kills.