Blades of Time

Here is a video to the method Wicelow mentioned for those of you who prefer a visual walkthrough.

Credit goes to the uploader not to me

The 1st encounter with Brutal Maul is at the beginning of chapter "Brutal Lands" near the end of the game.

After fighting 2 waves of the chaos ghosts thingies, a cutscene will play to introduce them.

Now slowly move towards them and as soon as you see one, shoot him in the face.
He'll use his attack (big freeze wave on the ground) but you're too far for it to reach you so don't worry.