I believe these numbers as very close to the percentage I put earlier. @Campo
10% chance you will get mimic battle from boss chest.
Another 5-10% chance you will get Hoard mimic battle from mimic battle.
Another 5-10% chance you will get the mythic drop.
These are based on my numbers. I done over 2000+ boss chests.
My friend @Cronus numbers properly more accurate since he done more battles and more data.
Good luck