You can get this achievement in the 16th level called "River". You can NOT get this in the level "Boat" when Crowe is attacking you from a chopper. This has to be done two levels later. Make sure that you have a Sniper rifle in your inventory during this level (you might be able to pick one up right before you're going up against Crowe)
After you've cruised down the river with your boat you will get into the jungle. Doyle will mention that Crowe is ahead of you. When you get to the area with Crowe you'll have to kill four soldiers at the beginning. Take them all down and then slowly move forwards.
Have your binoculars up to mark a large group of enemies. One of those enemies is Crowe. You can easily spot him because he's wearing his cowboy hat and he also has a different outfit.
If you approached him slowly he won't spot you and you can easily line up a shot with your Sniper. Other guns might work just as well but the easiest and safest way for a clean headshot (and one headshot will kill him, at least on Easy).