At the end of Act 2, the part called Last Stand, where you are to defend yourself from the Locust Forces, just as the Act is about to finish 3 boomers (thats the big guys with rocket launchers) will appear from an emergence hole behind you, from where the main action just occured.

Just above them is a large cylindrical object and you can see 3 white propane tanks.

Shot the tanks and boom crushed Booomers - no pun intended!

The moment that is done just reload the checkpoint and repeat for easy achievements.