The Hunter is a regenerating Necromorph that cannot be killed. The first time you encounter him is very hard, and you just have to run. It is best to shoot off his legs because he does not regenerate them right away. He is much slower without legs. Then, freeze him and he will be taken away. When facing him again in a later act, use the same strategy then fry him with the rocket engines. Collect the Power Node that it drops.