Well, as the tactical scenario seems to have deteriorated significantly. I'd hope that the already firing M14 rifle, aimed at the chest, and climbing towards the head, would work. However, we don't know the anatomy of the alien. Could be similiar to shooting a human in the hair. Only way to find out is to watch the movie.
Just out of curiousity, how the heck did the two of them get so close in the first place? :?