Bosses are powerful enemies built to beat the crap out of you. Most can be killed solo, but some may require a team.
Full Bosses are Bosses that have their own battle Arenas, suited to their nature. These often have more Armor, Weapons and healing than normal ones. They include:
Rae-Arma: General of the Harral Army. He's a powerful attacker, with weaker summons. He can summon Fire Demons
Zal-Veneno: General of the Zephyz Army. She will rarely summon Constructs or Rot Spiders in her Minion Phase.
Tir-Res: General of the Medasz Army. Very weak in combat, but his use of protection spells is unmatched. He will summon FAR more Mobs than any other Boss. He mostly spawns enemies for defense. These would include: Golems, Consctructs and Rot Worms.
Dra-Pacem: General of the Dramae Army. His common name is the Second Angel, being an Angel himself and Dramae being the first. He cannot be directly fought, his 3 Stone Guardians, Pax, Zal, Osim must be killed first. All of these must be killed in order to confront him. When you can actually fight him, he's quite a push-over. So, the only thing truly dangerous is his Guardians, which must be fought all at once.
Draca: Draca is a rare Dragon-Human who dwells in the Ancient Casket Room along with his two sisters: Kai and Inmortalis. They defend the Ancient Caskets, which can hold some amazing loot. You need four Key Shards to make the Key that unlocks Ancient Caskets. Once you have both a Casket and a full Key, bring both to a Greater God or higher, and they will open it. Once open, you'll have to fight one last enemy.