To join boss battle, you'll need 4 different pet type - Novice, Mage, Assassin and Berserker. It will costs 100 stamina and 1 FOXY per battle. You'll have 5% chance of obtaining NFT crates after each victory.
Boss battle results - 1 to 10k RNG + boss power
Your battle results - 1 to 10k RNG + sum of all pet's power