When you are doing Level 10: Reclaimed and the objective becomes "Destroy the AA guns (3 of 3)" you will eventually come across a soldier using a flamethrower to burn an alien.

When you do, you will also reach a checkpoint and a notification will appear on the left of the screen.

When this happens, go down the hill and kill everything you can see, then reload the checkpoint, and rinse and repeat (well just repeat).

You can get 6,500 XP an hour doing this!