Story related, can't be missed.
Sublime is the boss on Mission 4. He has 3 phases. The first phase is just attacking him normally. He will then go into his second phase. You will have to dodge him and attack him and he will jump up in the middle on a machine to heal himself. You must go to the various control computers around the room and press to electrocute him. In phase 3, you will have to dodge his attacks. He will then punch the ground and rest. Jump up the structure and attack his face. You will have to do this a few times. If you die on any phase, you will have to start the whole fight over again.