Play on the last level of No Mercy and pick up a pipe bomb at the start.

Go through the level normally until you get to the room with the radio.

Before you call the helicopter go to the roof and check the buildings to the right and left for pipe bombs (pipe bombs may not always be there)

Call the helicopter and go to the gun on the roof.

Throw your pipe bombs and kill the zombies around them.

Do this a couple of times and the achievement is yours.