In order to get this achievement, you have to deal a specific amount of damage to him BEFORE hitting him with the crane. Hit the button to move the crane, stun him with a flash grenade, then knife him until he goes down on one knee and keep knifing him for a while, it will probably take more than one knife to do this. You can tell that you've done just about enough damage when the eye on his shoulder turns red. Then run back and hit the button and flash him again when he stands up.