I like the liar idea as a concept, since it relates so well to the "evil" side of Dark-type moves rather than just the "lack of light" that doesn't embody most of them. As XTS said, since guessing wrong means you just have to fight the trainer, eventually you'll be led to Layla in the end. People might need the experience anyway and fight everyone else first since Layla's Pokemon are pretty high-leveled.