How do I kill ammit?

Ammit's like any other boss. You can try killing it in singleplayer with your family or join a server in multiplayer with a bunch of people and defeat it with them. If you're gonna go with solo, I recommend a higher level (150-200). You can find it in Ancient Egypt, second pyramid from your den. Once you enter the pyramid, look to your right and go through the third wall, counting from the end of the hallway, to enter its chamber.