Anzu is a Sub-Boss in Ancient Chaos located in D-2.
Anzu does not appear until you have put a weight on the dais between purple lazers, immediately below in D-3. If you do this before fighting Anu you will have to beat Anzu before you can reach Anu's Room.
Anzu will start not moving, and can't be hurt. After a while it'll move and freeze time rapidly, making it unbeatable. Using the Lamp of Time to freeze time will allow you to defeat Anzu.
Defeating Anzu awards you with the Destiny Tablet.