Yo dude, you don't need to fight all the bosses. When you defeat a boss, their crystal lights up on that big tablet at the end. You need to defeat bosses so that a there is a complete circle of light on that tablet, it doesn't matter where the connection is, it just has to be complete. So when I play the mission, I generally fight against Beowulf, Agni & Rudra, and Cerberus. I think that does it. There are other combinations to do, so choose whichever bosses you find easiest. However, there is a reward for beating all of them.