The Desert Mercenary by Barry Sadler (Casca Series, Book 16) from Books In Motion.com

Casca Longinus. Cursed by Christ on Golgotha. Condemned to outlive the ages, wandering the globe as a constant soldier. Forever fighting, surviving, unable to die, and waiting for Him to return. Casca finds himself caught up in the ageless struggles of the desert tribes. With a hardcore group of international mercenaries, Casca assaults a fortress on a mountaintop high above the blistering Algerian desert. The mission is to rescue the son of a wealthy munitions dealer and his wife. Even Casca's ""dirty dozen"" might not be able to overcome two hundred of the toughest men the desert ever spawned.