The Mastiff is quite agile and capable of jumping around the area with ease, including onto the ceiling or walls. Because of his size, reach, and agility, this is the point where you need to learn to Parry attacks because there's no other method that is consistantly good at defeating them. With the exception of their grapel attack, everything should be a parry.
When he's not attacking, hit him a few times. As you do damage, more and more of his limbs will start to glow blue. When they do, enter Blade Mode and cut them off. This will limit his ability to perform certain attacks. After enough damage, you can cut him in half like anything else. Just remember that you must parry because Ninja Run will not help you much against them.
