Dolmen

Pretty simple: after beating a boss you're able to respawn them from either the consoles outside their arena or from the ship; the process is the same for both. At the ship you'll take the elevator to the upper floor and "Interact with Seba" ... why this wouldn't be accomplished through the big, impressive-looking table to his side isn't quite clear.

By doing this or visiting the respawn console outside the boss arena, you'll get a menu that offers
- Respawn Boss
- Multiplayer
- Quit

Select Respawn Boss and have at least three dolmen fragments in your possession. If you're on the ship you can d-pad up to the boss name and then scroll left or right to choose a different boss; if you're at that respawn console outside of the boss arena you can d-pad up but will be unable to choose any other boss.

At this point have the boss you want to respawn selected, d-pad down to the Activate button, and select it. The boss will be respawned and you'll immediately be taken to their arena and the fight.

Achievement Unlocked.