There are two different bosses in this game. One you will find after you clear every 5 normal quests, another one during an event.
For normal quest boss fight, you can get help from two friends, one from each element different from your chosen element. After the battle, you will win some rewards, coins and Energy Drink/Power Potion/Trap.
For event bosses fight, there are two types too.
One work as normal quest, you will face them after clear every 5 event quest and get some rewards, you can ask help from two friends.
Another type is raid boss, which will turn out during the quest by random. It normally started with 500 HP, gradually increase up to 500k HP. Sometime you will have to fight against event king/queen boss which has more than 1 million HP. Current even Hades’ wife has 10 million HP and is super hard to kill. During this kind of boss fight, after 1st fight, you can choose to continue fighting alone, or ask for help from 9 friends/guild members. Unless you spam your PP, no way you can defeat the King/Queen boss alone.
- Boss Battles: Boss battles are the end point of each quest. These are tougher fights, with the bosses actually able to attack you. In these fights, defense is much more of an attribute that you will want to take note of before heading in. However, when you finish off a boss, it typically will give you huge experience bonuses, as well as a treasure trove of goodies dropped (energy drinks, coins, items that enhance your cards skills).