Quite simple this one...
There will be moments where you'll enter shooting scenarios during your run. The enemies will run into your view and will then get themselves into position. Once they stop moving, maybe a second after, they will either chuck a spear or shoot an arrow at you. Time it right and tap on the projectile as it comes toward you and Lara will shoot it out of the air.
This is cumulative and does not need to be done in one run.