Shoot the red pistons to damage the massive crane. Also shoot the helicopters to avoid losing too much health. If you're near the end of the fight and want to take out the crane quick do to limitations such as health, or just your own pace, you can avoid the helicopters, saving time. However, consider you're current situation when doing so. It would make sense if the crane can only sustain a little bit more damage, since there's less time left to take hits yourself. After going back and forth between two pistons as you circle around the crane, you will have to destroy one more piston. Then, there will be a QTE in which you kill the enemy operator. You have to press the buttons exactly when they pop up, or else you'll have to wait for little more time before the opportunity comes again. After this, a cutscene happens and you unlock the achievement.