There are 9 bosses total as of now, the last ones are only unlocked above certain difficulties:
Bosses have deadly attacks, you dodge them by walking, using your R movement ability or your 
Mystic Guard invulnerability. At 100% energy bosses use their ultimate. There are ways to hinder or even completely deny each boss energy gains.
Bosses will get more dangerous as they get closer to death, they can have stronger final phases and unlock new moves. Burst spells, souls and scrolls should be saved for the end of the fight.
Vulnerability[]
Bosses have a vulnerability multiplier, starting at x1.00. Damage dealt to the boss and his minions gets multiplied by vulnerability, though some items and souls are not affected: 
[Arcana Potion], 
[Pandemonium's Void], 
[Spirit Fury]. Some characters have spells to temporarily increase this vulnerability, depending on the difficulty it can also increase over time:
- Easy: often increases by x1.00, capping at x4.00. Resets when boss health goes below 80%, 60%, 40% and 20%. For Rayel and Narthuul resets every 50% instead.
- Normal: similar to easy but caps at x3.00 and increases less often.
- Hard+: doesn't increase or reset.
Impossible: bosses have slightly more life and all their attacks do 25% more damage.
Counter[]
Some classes have a counter ability on F which can prevent the boss from casting specific moves. The counter priority can change at any time if the proper counter class is out of range or unable to cast spells.
- Tanks is melee and low cooldown, he should counter slow or predictable moves: Root, Deluge, Lovesong/Demoncall, Hiddenfrost, Renewal, Anomaly, Sleep/Nightmare, Stoneguard/Soulritual.
- Warlock/Dryad is ranged and low cooldown, he should counter quick or channeled moves where tank would be too slow: Ancient Blast, Overflow, Bewitch/Moltenrush/Firetrap, Coldsap, Gravelings/Vanquish, Starcrush, Mindrot, Ether/Stoneguard.
- Ranger is ranged but very high cooldown, should be saved for bad situations. She can also use Preparation to reset tank or warlock/dryad counter.