Berserker Brother Assassin (Saberhagen's Berserker Series Book 2)

Not satisfied with efforts to destroy all life in present time, the Berserkers have devised a way to penetrate human history. The death dealing machines pinpoint key personalities in human history for destruction. When and if they succeed human progress is set back perhaps thousands of years.
Humanity has an answer. On the planet Sirgol, the only planet in the galaxy where time travel is possible, an elite corps of Berserker hunters struggle to derail the Berserkers efforts.
Lieutenant Derron Odegard of Time Ops faces the death dealing Berserkers in three exciting adventures: Stone Man, Winged Helmet, and Brother Berserkers.