Really loved that one. Invisible enemies that can be detected only for a brief period of time is absolutely fantastic!
I had a problem with "gun". So you have to charge it up until the moment it becomes deadly, but if you overcharge it it misfires. And the only clue of this charging is audio one. And the enemies are detected by the audio clues.
And the "kill" window of a "gun" ~1 second. So, i have to count to 5 every time and focus on the alien sound to kill some aliens. But you have to kill only 6 in 3 rooms so it was not so bad.