Assuming you are targeting the right boss, his ammo pack is on his butt. I'm reading that the Firefly skill that destroys weakpoints is good to use here if you have trouble registering the objective the normal way.
In addition to what SevenNVD said, make sure you have field research set for technician and that the difficulty setting is at, or above, the level specified for the task.
The riot foam is also a great tool to make sure he stays in place as trying to destroy his weak point is difficult due to it being positioned at his back