The greatest difficulty in fighting the corrupted custodian is that the boss is really tough. As a result, you'll have to make use of two of your best attacks, and be aware of the fact that he will be losing his health points very slowly. Unfortunately, you cannot count on Karn's help with his battle. As far as the boss's attacks are concerned he, above all, uses his upper limbs, both for the fast attacks and for attempts to crush the Death.
Another one of the boss's attacks, used pretty frequently, is a ground shaking punch into the ground and you should already know perfectly how to deal with that, jump that is, and try to move away from the custodian as quickly as possible.
Definitely the most difficult to avoid of the bosses attacks is a huge vortex that moves towards the Death. In such situations, do not let yourself be cornered, because this may cost you a lot of health points. As soon as you see the vortex try to move far away from the custodian as quickly as possible, dodging whenever it's necessary.
The corrupted custodian should be attacked only when he finishes one of his slower attacks, but remember, at the same time, not to deal too many blows, because he may regain strength and respond to your attacks.
If you have not yet used the Reaper Form, you should definitely do it now, although you should not expect to deprive the boss of the entire health bar. In a further part of the battle, you should also wait for the situations in which he stops. Then, run up to him to launch the best of your attacks.