At the end of each Act, you will encounter one of the bosses for that act. The boss is chosen randomly. The boss fight is denoted in the map by the large shield, swords, and skull icon. The preceding space on the map will always be a Camp Fire, allowing you to rest before the fight.
Each boss slain adds 50 points to your Score multiplied by the total number of bosses you have encountered. Slaying a boss without taking damage (not including self-damage cards such as Offering, Hemokinesis, etc.) adds an additional 50 points, and perfecting all 3 bosses adds a 200 Points bonus, overriding previous flawless bonuses.
If you have Pantograph at the start of a boss fight, it will heal you for 25 HP.
On Ascension #20 (and beyond, if added), you will fight two bosses at the end of Act 3. You cannot fight the same boss twice.
In the Mind Bloom Event in Act 3, choosing [I Am War] will allow you to fight a random Act 1 boss for a random rare relic. This boss fight does count for the Score bonuses and does activate the Pantograph relic.