Manic LumberjackG 15

There are 2 ways of earning this achievement:

One way is to use the mortar device during 'Crossing Over' after you reach Haggard and start getting bombarded by snipers hiding in small forests. Just kill an entire forest where they are hiding.

OR

In 'Air Force One', at the beginning of the level just after the Heli tutorial, there are small forests that are river-side and below you. This is a much easier way to get it. Just clear out a small forest and the cheevo should pop.

If it doesn't pop, try a little bit bigger of forest.