Shadow Man Remastered

Although the description makes it sound like there's going to be numerous helicopters, there are in fact only two of them.

They both appear during the Liveside level Gardelle County Jail, which is accessible by putting a retractor into the chest of the body hung through the door which is straight ahead of you when warping to the Cathedral of Pain. The symbol above the door looks like a Lizard.

Follow the level through as you normally would, collecting the keycard and making your way through the prison grounds.

The first chopper appears in the first large open air area you come to. Equip your best guns / voodoo and fire away, taking cover behind boxes when you do so to avoid its machine gun fire.. Don't get too close to it, as it explodes when it's taken enough damage.

Continue following the level until you get to another larger open area - the second chopper will appear.

Again, unload with your best stuff.

When the second one goes down, enjoy your achievement!