This page of our FNAF Security Breach guide is dedicated to Monty boss encounter.
Unlike the rest of the animatronics, Monty will attack you before he is decommissed.
During the fight, all you have to do is run between the turrets and fill the container by firing from them. You have to do this while staying away from the boss. He will chase you most of the time, but if he loses sight of you, he will start jumping at random points in the arena. Unexpectedly, being chased is better than not, as his lunges are really random.
Morever, don't forget that he is blind-proof.
The best tactic is to stay in constant movement. Stop only at the turrets, and only if you are sure Monty is far away from you. Also, don't stay too long at one position and relocate after firing 2/3 shots.
The fight will end when you fill the container (which takes about 30 bullets to complete).