For this you must kill 5 Behemoths or Mirelurk Queens. Those are the strongest enemies in the game and they drop some rare loot. Bring a Mini-Nuke Launcher (Fat Man) or Rocket Launcher and also wear Power Armor. Otherwise they might one-hit kill you if you are underleveled.
These boss monsters don’t respawn. You will also encounter some of them during quests. As long as you land the final shot it will count towards the trophy / achievement.

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