Hit the juggernaut
The Juggernaut is a higher strength enemy which appears in Call of Duty, in Modern Warfare 2 and after. Its strength may vary depending on the weapon it is shot with (the most effective is the Barrett .50 or other sniper rifles, or the Desert Eagle).
What makes it different as an enemy is its reinforced heavy armor. The difficulty in killing it depends solely on the weapon you use and what part you hit.