I got this in Chapter 2. Right at the beginning, there are cars all over the road that you can shoot once or twice with your pistol (or slash with the katana, if you want
) to cause them to catch fire and then explode. Just try to time it when a few of the enemies are near the vehicle, and you can easily get a few kills here.
In the next section (inside an arcade), you can blow up some of the arcade machines (the ones that are lit up the most, usually near the end of the rows). Just deploy the same tactics here, and you can get a few more.
There are also explosive pots, barrels, electrical boxes, etc. throughout the game that you can use for this, but I found the beginning of Chapter 2 to be the earliest place to get a lot of kills, quickly. After the arcade, you can just die and start the level again, racking up more kills with environmental damage, until the achievement pops. Good luck!