-( Story related, cannot be missed )-
Chapter 3: The fight with the Fighter robot will play out much like the previous two. Kotoko's robot is an axe-wielder, and the entire arena is flooded aside from three elevated safe points. This means that you will be relying heavily on the PARALYZE bullet you obtained earlier in the chapter. Once the robot has taken a swing at you and stalls, the axe lightly touching the water, aim at one of the random Monokumas that will also be in the water and use PARALYZE. This will stun the robot and expose the weak spot, which happens to be in its chest as well. You will be repeating this tactic throughout the fight, even if it does change up its attacks once the chest plate's covering breaks a bit.
When I mean change, you will probably start to get hit a bit more often once it moves into its second phase (or I could just suck at avoiding it). It will do a three-hit combo before stalling on the third hit, which is your opening to continue on with PARALYZE.
NOTE: You can continue to obtain experience from the wasted Monokuma in combat, and the amount of PARALYZE bullets dropped by them seems more than abundant on the normal play mode. Just a helpful hint! ;)