This one is a bit though and requires at least having cleared the first four endings 5 times, preferably also some pride runs and you'll need to have a max destiny of at least 32k when starting the Evil City.
The Corrupted Great Bird is in the boss dungeon of the Evil City, which should unlock after finishing the Holy City.
The bird is weak to Earth Damage, so if you are magic oriented be sure to equip some relevant skills. Focus through Gluttony on increasing the Magical Attack and Magical Defence. This may take a few Dark Rituals.
If you are on a grind for gluttony, your Guardians Bane should also have increased a lot. Meaning that it should be possible to unlock the last guardian and use those summons in the battle against this boss. Don't worry on spending Sin and Inspiration to unlock the 5th and last guardian, it is well worth it.
Invest a lot of your habbit into familiars so you have at least 50 familiars summoned which can then all be assigned to Greater Demon.
Play around with party templates to see which party configuration will deal most damage and don't forget to check on magical effects like the Silver Scale Dragon and Scarlet Dragon.
Somewhere around 30% of health left, the bird will try to heal it self which should give plenty time to finish him of. Make sure your last ally does a lot of damage and have your first two allies do quick attacks. Don't forget to enable both Friendly Formation and Magical Formation.
Lastly don't forget Envy and you title to maximize the damage.
It might take a few tries because his attacks are random. You need to be a bit lucky for him to start of with two longer attacks. If you can't get him to 50% in the first 5 attempts, go grind some more Gluttony, Familiar, Envy, Ally levels and Status upgrades in the Astral world.