This trophy is fairly simple, and can be obtained as early as Earlston Yellow Zone. You can either use Firecrackers near KPA forces, or silently take them out from a distance.
I recommend equipping a Crossbow with a 4x Scope, and snipe KPA troops near guard posts. A good example is the Fuel Depot - climb the northwest building close to the fuel depot where the gantry walkway is, crouch and take out some enemies on the stairs/shoot bolts near the area.
Rinse and repeat 30x times.