I would suggest turning several of them into neutral clerics of Wee Jas, the neutral goddess who has no problem with raising the dead as long as you're not all evil about it. In the weke before the big fight, have them go out and use liberal rebukes to round up as many undead as possible. Zombies and skeletons can be rebuked with pretty good regularity at level 1, each cleric will have 4-5 rebukes per day, and they will follow and obey the cleric who rebuked them until they die (again).

You are going to use these undead as meat shields against the aliens.

Whoever isn't a cleric should be rogue archers or slingers on the rooftops/walls with a sprinkling here and there of marshals, bards, or dragon speakers. I prefer bards, myself.

Your clerics probably won't be able to rebuke the aliens when they die (too high HD, I'm assuming), so use them to rebuke any villagers who get the axe into fighting on your side, and send them into the melee. Besides that, have them bless the archers, use inflict spells to add what damage they can, and be ready to protect the archers as much as they are able if the aliens make it up the walls.