You’re the Boss(Slinger)

Achievement Unlocked: You’re the Boss

Defeat your first boss! Hopefully the first of many!
 * You will now see a boss’ loot drops upon using your Observe (or its variants)
 * Enough XP to level up!
 * Guaranteed Legendary (already awarded)
 * You can now see the health of a boss with Observe (or its variants)

Reference
Unlocked in Chapter 17 "Ready? Fight!"