-( Story related, cannot be missed )-
Chapter 4: Nagisa's Sage Robot deals with a long-range attack pattern, always trying to keep its distance from you and working with its rifle instead. The weak spot this time is the head, though you will be told it's the eyepatch (the hit box is actually surrounding it). Take shelter behind one of the tower blocks on the battlefield until it is on the outer edges of the arena and fires off three shots. After the third, it'll start to reload, giving you your opening to fire off a shot to stun it. In stunning it, you will be given the chance to fire four BREAK shots on it before it resumes its attack by jumping into the center and sniping again.
Phase two will start with him moving to some windows to the side of the arena, distancing itself from you and increasing its speed. Along with taking a few shots before moving, he'll also start throwing a grenade or two out. The same tell applies: when he goes to reload, take a shot, but that's the only one you'll be getting off. Repeat this a few times to make him move back out into the battlefield.
For me, after he did this, he blanketed the entire inner ring with grenades, only for him to end the fight after that, so... I'm not sure if my BREAK was that powerful to promote such a thing or if he just went rather kamikaze on me. Either way, the tactic is to shoot his head to stun him and then dish out some damage. Depending on what is more powerful, you might want to see if BURN could deal more damage (with its higher capacity and faster speed, it might help, but I can't confirm this).