How do you beat the last level on sierra 7

You have to basically peek out of cover and line up your aiming reticule with enemies. Then at the minigun part, you have to shoot him when he stops shooting. LISTEN CAREFULLY TO THE BULLETS. if done correctly, Sierra will start moving each time a certain amount of health diminishes from you shooting the gunner. Kepp shooting then youll find the guns and then blow it sky high hope this helps :))