Boss Batles typicaly occour every 5 quest levels. A Boss Battle will award typicaly Coins, a Trap, and either a Power Potion (maybe bound or not bound) or a Energy Drink (maybe bound or not bound). Event quests typicaly will award a Boss Battle every 5 quests up to Quest Level 100. After that Boss Battles will cease to exist. The main quest there are 25 total Boss Battles.
These battles are fought with your main Cryptid. You can also choose two other cryptids from your friend lists. Boss Battles' eventualy get so hard that this is not possible to without your two friends cryptids. You can use power potions during the Boss Battles' to replenish your defence.
The battle happens in 2 stages. Stage 1 you and your friends cryptids attack the boss. Stage 2 the Boss attacks you and your friends Cryptids. The Key is to get him down to 0 health and win.
Occasianly their is a hidden reward after defeating a boss or a event boss and you get a free rare card.