Brutal Lands

Kill the Vicar of Chaos.

This is story-related and cannot be missed.

This is the boss fight that ends chapter 8, Brutal Lands.

At the start, the Vicar will be powered by two Brutal Mauls; the power he gets from them enables him to use a spell that sucks you up to him and lets him throw you down to the ground. Use time rewind to create a clone of yourself that the Vicar will focus on, then get to each Brutal Maul and kill them (they're much easier to kill than usual) - this takes away the "vacuum" spell.

Now things are straightforward: use dash to fly to the Vicar and hack at him; when he uses his "spinning blade" attack, simply put enough distance between the two of you to be able to turn and shoot him with a machine gun (you can use time rewind in order to do more damage to him, but beware: if you rewind time too far you'll actually restore health to him). Alternate between these two attacks - dashing and shooting - and the Vicar will be easy to deal with.