Hell on Earth by Dafydd ab Hugh (Mass Market Paperbound)

"They were creatures seemingly spawned straight from the pits of Hell--demons, zombies, fire-breathing imps--all too horrifically close to the stuff of nightmare to be real. But they were. And on the inhospitable moons of Mars, Corporal Flynn ""Fly"" Taggart, Earth's last line of defense against a seemingly inexhaustible supply of alien warriors, beat them back almost single-handedly. But Taggart discovers that the war had barely begun...for while he was fighting them on Mars, the hellish creatures had established a beachhead on Earth itself. Now, with the aid of a fourteen-year-old female computer genius, an unrepentantly Mormon sniper, and the best soldier in this woman's army, Fly Taggart must defeat the invaders--and their treacherous human allies--yet again..."