Chances are you'll be around half-way to getting this by the end of your first play through.
Even if you're not, the mop-up is quick and easy.
Load up Chapter 10 - The Customs House. After the first cut scene you'll head up some stairs into an area with two big staircases and some baddies. There's an M79 baddie on the stairs in the far corner.
Take out all the bad guys then grab the M79, but don't use it yet. Open the door using the chain winch and after the next cut scene, use your pistol to pop the padlock of the door and jump onto the balcony.
After the QTE with the falling balcony, three dudes will pop out inside the building. Use the M79 to take them out through the grill/window, then restart the checkpoint. You'll jump back to just after the QTE, with the M79 and full rounds.
Rinse and Repeat!