The Hunter is a regenerating Necromorph that cannot be killed. The Hunter is difficult to defeat the first time that you encounter him but can be done by staying away from him and shooting off his legs because he does not regenerate them immediately and it will slow him down. Then if you freeze him he will be taken away. When you encounter him again in a later stage use the same strategy and fry him with the rocket engines. Collect the Power Node that it drops.
The following trick can be done when you are out of ammunition but have a bench and several Power Nodes. If you upgrade to a capacity node in a weapon you can refill the ammunition in it to it's new maximum. This will also restore your health and help save medpacks.