I'd suggest:
1. Have an escape route for the people. Even if it's just a ladder to climb off of a building, it's better than being trapped in a cellar. Unless these xenomorphs can climb walls . Anyway, try to bring some of the noncombatants along, if you can. A few crying children should melt the heart of any real paladin.
2. Make lots of villagers into improvised clerics, if you can. Turn undead and protection from evil can really help against massive waves of zombies.
3. Try and force the kytons to fight their undead ex-chums, if you can. If you can do this, It'll drastically weaken the forces of both sides.