The boss is a quite a demanding adversary, but if you have well tuned equipment at your disposal, you don't really need to be too afraid of him. The boss attacks with upper limbs, and unfortunately, these aren't blows into one direction, but also to the sides (the above screenshot), that cover a larger portion of the arena.
During the battle, you'd better not remain around the circumference of the arena, because, he can reach you there also. It is much better on the idea to run around the boss and keep him at a distance (the above screenshot), thus making him perform troublesome turns.
Throughout the entire battle, keep trying to find a vulnerable spot around the head of the boss shown in the above screenshot. Strong special attacks may turn out to be very useful. Reaper Form will not be effective in this individual case, because the battle has been divided into many pre-defined activities, and that is why you can't deprive the boss of a large amount of health points "at the first try".
Continue attacking the head of the boss and avoid his blows at the same time. It do so until the boss falls on the ground. Run up to his head quickly, and initiate strong blows, which will promote you to the SECOND PART of the battle.
[TIP: there are a few moments during which the boss falls to the ground. When this happens, you need to act quickly and initiate the above mentioned activities, or else the boss will regain the portion of his health, which will result in a longer battle.]
There is a cut scene ahead of you, during which you will not be able to the deal damage to the boss. Instead, you are up for mass attacks from spawns. To eliminate small groups. Use standard attacks, make use of wrath points and stronger attacks only when you are surrounded by a larger group of monsters. Defeating spawns will promote you to the third part of the battle.
The third of part of the battle is very much like the first one, so we'll need to aim at the boss's head and avoid his attacks. An additional obstacle is the necessity to fight off new waves of spawns. Fortunately, this is not going to be a problem.
Continue initiating attacks until the boss falls to the ground again. Run up to his head quickly and pres E key (just like executing regular monsters), and remove his protective mask. This way you will be promoted to the FOURTH PART of the battle.
The fourth phase is somewhat similar to the second one. The only difference is that, apart from spawns, you will also have to pay attention to the symbols on the ground (the above screenshot). Keep jumping away quickly from such spots in order to avoid taking damage. Attack the spawns until the moment that the boss returns to the arena. This is the beginning of the FIFTH PART of the battle.
In the fifth part, apart from avoiding the boss and the spawns, you still need to keep distance from the blue symbols that appear on the ground. Just like before, focus on attacking the boss's head.
After he's taken enough damage, the boss will again fall to the ground. Quickly run up then to his head, and press E key to end this battle.