Boss enemies are first encountered during "arcana release" quests unlocked for finishing all (or most) of the quests in a chapter. In boss quests, the boss monsters won't appear on the field like normal enemies, they will be represented by an otherworldly portal that usually move around the zones of the map. Around the time you first face a boss, they will be stronger, bigger and have more health compared to regular enemies; they also have more variety of attacks. Most boss enemies have multiple areas to target that appear with a blue lock on recticle , these areas when attacked consistently will eventually break off and be dropped as loot; this can also limit the abilities of some attacks or stop them from being used entirely. Around half way through a boss monsters health, the camera can switch to the angle normal mosters get before a coup de grace, if attacked while locked on, you can trigger counter duels which involve button mashing to fill a meter. At the end of them, the boss can drop loot and takes damage depending on how high you filled the meter. Around this time, bosses can enter rage mode, they enter with a large roar that damages all nearby players, this also makes them look darker and their eyes will glow. While in rage mode enemy attacks will be more damaging, the boss will be quicker and most bosses gain new attacks or stronger versions of old ones. Rage mode lasts a few minutes but can be entered multiple times during an normal boss fight. When at critical health, the boss monster will bleed and start walking towards the red circle to escape, if they succeed, you are send back to the field and the otherworldly portal is moved. When found again, the boss will come back with a bit more health. When their health is all gone, a melee duel will begin, this is a standard quick time event set into 4 sections (one for each player in a full game). You have to press the buttons as they appear on the screen in a small amount of time, if failed, the boss regains some strength and you continue the fight. Boss monsters are defeated when the melee duel is successfully done.
Boss cores are usually required to make stronger weapons and armor. They are also used to make boss cards which you use for ultimate spells.