Load up Executive Order. Once you've secured the comm tower, you will get a checkpoint. Right afterward, you'll be given the crossbow with the explosive bolts.
Two trucks will pull up with 3 guys each. Kill them. Then in the distance, several enemies will come from behind the building. Kill them all, including the guy on the truck with the mounted gun.
When you are instructed to shoot the zipline to Weaver, instead kill yourself or reload the checkpoint. You will start again at the top of the comm tower before you get the crossbow. Rinse and repeat.
I had to play it 3 times to get it, but you might get it the second time.