Right at the beginning of Chapter 10 you have to protect Chase using the Dragon Sniper from enemies coming down a hill and also stationary snipers on wooden towers. Chase is on the ground in front of you and moves from cover to cover with each wave - you are on a raised platform. This is a good point to just farm the 50 kills needed for this trophy.
It will take a few goes kill all but leave one to kill Chase, then repeat until it pops.