Defeat Grand Unifier Raker.Story-related; cannot be missed.
This fight takes place right at the start of Chapter 15 and is another pretty simple one since there are no other enemies. Raker tends to just hover and shoot from above, so his attacks are all easily avoided. Just be careful of when his projectiles rain down (stop targeting him and focus on the circles on the floor to dodge them) as they can do a ton of damage. Focus fire on him to remove his shield and stagger him, then open fire to damage him. He has three health bars so don't let up or slow down, and he should go down pretty easily.