The easiest place to get this is by replaying Chapter 10-Part 2 over and over again, as you use a grenade launcher the entire chapter. But while you're playing, you can also use the M249 SAW, which is always dropped by the Heavy. This will be the most common heavy weapon you can use throughout the game. The SCAR-H also has a grenade launcher attachment (by pressing up on the D-pad) that counts, though you won't be able to use this often as it only usually has 2 grenades. Then there is the RPG (which is in very few chapters) as well as the grenade launcher (which, besides being in Chapter 10-Part 2, I only found in Chapter 4 and 14) as well as the AA-12 shotgun (again, that I only found in Chapter 14). So play the game through, making sure to use heavy weapons as often as possible, and if you don't have the achievement by the end of the game, just replay Chapter 10-Part 2 until you do.