Encounter: Drawbridge (Chapter 10)
First you should beat the story to unlock weapon select and the infinite ammo cheat. Without infinite ammo this will be very hard to do because the grenade launcher comes with only 3 shots.
You can buy cheats from the bonus menu and they don't disable trophies.
Now load up the Drawbridge encounter in chapter 10. It's important to note that the China Lake Grenade Launcher can only be selected during certain encounters. The Drawbridge encounter is one of them.
Activate the infinite ammo cheat and use weapon select to get the China Lake GL. Now shoot the enemies and you should get this easily. You can kill 10 of them in less than 20 seconds. Without the infinite ammo cheat you would run out of ammo too quickly.