Path of Exile

Malachai or in this case, Malachai the Nightmare, is the final boss for Act 4 and can be found in The Harvest.

Malachai resists elemental and chaos damage.

Malachai has two stages to pass in order to defeat him

In stage 1, Malachai will appear with a red shield that will block all damage. Piety will help you by bringing down his shield using a beam. Try to avoid this beam if you can. When Malachai's health drops to between 75 and 50%, he will transform Piety into an abomination and she will now start to attack you. Malachai's shield will return. You will need to subdue her back to her original form and she will become an ally again and work to remove his shield. You will go through this process a second time upon which Malachai will destroy Piety and you will move to stage 2

In stage 2, Malachai will have 3 hearts of the beast around him. They are each protected by his red shield. At 75%, 50%, and 25% health, he will teleport to one of these hearts and steal the shield from it. This will allow you to attack and destroy the heart. Once you have destroyed the heart, Malachai will lose the shield and you can continue attacking him.

Throughout the battle, you will need to stay on top of which figure to focus your attacks to be able to work quickly. Malachai's various attacks are well telegraphed through the animations related to each type of attack which should make it easy enough to avoid most of the damage if you learn his patterns.