Stop an AST with an EMP grenade.

Missions 7, 9, 10, 12 and 13

An AST is a man wearing a big mech suit equipped with a gatling gun. They take a lot of damage before dying, and they deal a lot of damage to you. They first show up in mission 7 but can be found in later chapters. All you need to do for the trophy is throw and EMP grenade at one. It'll fall to its knees for a few seconds and damage it slightly. The trophy will unlock shortly afterwards. If you are having trouble identifying an AST looks like, please refer to the video below.