You can easily grind this on
Episode 1 New York, Chapter 3: Hell and High Water . As soon as you start the level, exit the train station area down onto the street. You'll have to clear out a few zombies. Once the streets are clear, flank to your left, near the end of the street next to a white car you'll see a breaching charge that will ALWAYS spawn there every time you play this chapter. Once you picked up the charge, continue through the level until you get to the first ammo supply section. To the left of the ammo supply is the breach charge door. Simply place the charge on the door, and boom, that's one out of 15 done. Simply quit and restart, and grind that 14 more times