As I said, this scenario is Alien II. The commando's battle, they brutally reduce the aliens, but in the end, only Ripley and Bishop survive. I REALLY recommend you lure the foes to battle each other, and also, ask your DM if you could reduce everyone's level by 1 to get one antilife shell. If it lasts long enough, you'll be able to kill the Kythons when there are very little of them, and next to no zombies. Else, prepare an escape route, 'cause you're going down.