Because the other solution is a joke, I'll tell you where thishe achievement occurs. This achievement is titled Into the Unknown and can be gained at the end of the level Argent Tower. After working your way up the elevators though the tower a cinematic shows Olivia diving into the portal. Waves of enemies appear and I survived a countdown. I didn't need to kill all of the enemies and the screen faded to the results screen at the end of the countdown. This achievement is story related and difficulty does not matter so it cannot be missed.