To get this achievement, you need to defeat three optional bosses called DELVER ECHOES:
DELVER ECHO ALPHA
DELVER ECHO BETA
DELVER ECHO GAMMA
You will first be able to encounter them after the main world changing event (defeating Major General Ezra Shields).
The Echos are better, stronger and faster versions of the Delver boss you killed earlier in Gideon's Rock. On the other hand, each of them is only one of the three phases of the original Delver boss.
Delver Echo Alpha (1st phase of Delver boss) will attack you if you come close to a half-buried chest in a large cavern in the underground (soon after defeating Ezra Shields, right before you reach Creo Institute of Technology).
Delver Echo Beta (2nd phase of Delver boss) - the same location, after resting in a MedBay
Delver Echo Gamma (3rd phase of Delver Boss) - A small courtyard next to Gideon's Plaza, where you originally found the drone cargo for the fake Russian doctor.
When you first come to the underground cavern, you might not be strong enough to face the Alpha. The good news is:
All three are available until the very last boss in the game! So there's no rush, if the challenge is too big, just come back later when you've levelled up a bit and upgraded your weapons.
Alpha and Beta won't attack you if you steer clear of the half-buried chest. Gamma will attack you if you enter the courtyard, so to avoid him, take a detour (through the underground).