Bosses open the map
Each boss unlocks an altar tier or a material your build needs. They are steps, not side quests you can skip for long.
The armor database shows the class you should wear for the boss you are about to fight.
Prep the loadout
Read the boss, then bring the armor class and buff that answer it. A resisted boss falls fast; a blind one drags for ages.
Bring healing
Boss fights are long. Carry more heals than you think, because the Shroud timer still ticks while you trade blows.
Learn the pattern
Bosses telegraph. Die once to learn the tell, then the second attempt is a routine you can repeat with confidence.
The consumable database lists the buffs that make those long fights winnable on your first real try.