This is the very first boss in the game, he shouldn't be too difficult to take out.
After picking up the devastator, and taking the elevator up to the football grounds, empty it into him, (don't forget the blimp) Run around until red smoke appears and you can recover your lost ammo.
Rinse - repeat, he should only take two - three rounds of this.
Once he's out of health, approach him and activate the QTE, hammer the button/key, and then score those sweet sweet three points.
The achievement will also unlock here.