Midnight Fight Express

I found mission 27 (Church) to be the easiest to farm for explosive kills. You have access to RPG weapon and 6 Gas Can throwables to use on 1st part of mission making it a great place to farm.

Going clockwise, starting from top right of map you can get:

RPG & Gas Can #1. RPG is leaning between stairs and ladder. Gas Can #1 is to the left of white van.

Gas Can #2 is on bottom right of map right behind a white car, next to a flipped green van.

Gas Can #3 is on bottom left leaning against black jeep.

Gas Can #4 is on top left of map next to door and yellow van.

Finally, Gas Cans #5 & #6 are in middle of map near the gray van.

Once you get kills, "Restart Level" to save progress and repeat.