Doctor Who: The Lonely Assassins

While the objective is not that difficult If you click and search everything, it's worth noting that the "point of no return" occurs when you send the clue "Flint's Bad Side". You have to have completed all tasks before this, because afterwards you wont be able to send any more clues.

The only remotely challenging part is "A creature on the phone! Delete all the corrupted fragments to destroy it!" – it's a timed challenge, so you only get one shot at it. You'll need to start the game again if you failed and Osgood had to take over. The 4 corrupted files are a photo, an email, a text, and a website. The website is a puzzle with limited time. The main trick is that after clicking on the moth, you need to click on the word CUBE instead of on the cube itself. For the rest of it, you should be able to deal with it without running out of time. Just keep finding the corruptions and clicking on them.

Other than that just make sure you read all the forum posts and download the additional file you get when asking Marguerite to resend the video to get all four TARDIS sightings.

If you've managed it, the achievement should pop after the post credit scene.