For this trophy, defeat 20 enemies using C4. The APC (armoured truck) encounter in chapter 6 might be the best way to do this one – there is C4 lying in a bunch of boxes all over the place.
Use the C4 to take out the APC, and then head for the stairs leading to the upper platform where a truck will spawn with reinforcements. Throw C4 at the truck and detonate.
Reloading the checkpoint will immediately spawn the truck again, so do this as many times as necessary until you get the trophy.