I agree - by putting "in full fierceness" at the end of the sentence, it's unclear which animal is the fierce one!
For example, a line from a famous Monty Python skit reads: "...foreign nationalists frequented the streets - many of them Hungarians (not the streets - the foreign nationals)."
My rewrite (good as any I hope): "The lion hides out, with contained ferocity, waiting for deer."
So you get the idea that the lion is fierce... but not showing it just yet.