You can farm this during the APC fight encounter in chapter 6 as this gives you enemies and C4 in the one place. You will find C4 in some lockboxes hidden around the area that you can pickup. If you go to the stairs where Nadine is waiting for you, you can desctory the APC. A truck with 4 enemies will appear by the stairs and you can throw C4 at it and detonate it to get 4 easy kills. The APC doesnt count as a kill. Die and repeat this mission over and over until the trophy unlocks.
Defeat 20 enemies with C4