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.

The lion hides out and waits for the deer in full fierceness. might be ok but The fierce lion hides out and waits for the deer. is correct although it may not capture your sentiment.

that's a tough one, because neither waiting nor deer could really be described as being fierce. So: two or more lions:"The lions hide out and wait to attack the deer" one lion: "The lion hides out and wait to attack the deer" or just maybe: The fierce lions hide out and wait for deer"