First, you will need some animals. In CryptoServal, you can only get them from other players.
Click here to get some:
To move to a boss, click on your animal on the map in order to select it. Then click on active boss and press the "Travel to .." button,
Once you have your animals in a proximity of the boss, click on the boss and press the Fight button.
Each team can have up to 3 animals. Choose your team wisely.
Your animals always receive some XP from a fight.
But if you win, you also receive gems + small chance to receive an accessory for your animals.