If you want to save ammunition (even though I wouldn't suggest doing this), you can kill Plant 42 with the knife; rather than with a Shotgun (or other such weapon). You must slash the vines which it attacks you with (eg. the ones that release acid, the ones that slash at you). Slashing the final one or two vines is quite difficult, and you will undoubtedly lose a lot of health if you have trouble dodging Plant 42's attacks!