This Achievement requires you to complete the game in 2 hours. You can play this under any difficulty but obviously the easiest way is to play on the lowest difficulty.
What it doesn't tell you is THIS HAS TO BE DONE IN ONE RUN.
You cannot for what ever reason stop and go to the main menu. If the game crashes (which it could the game is still a bit buggy) then you need to start over. I suggest being patient wait for certain areas to load. (I had the game crash after the bridge in the dam when you go down the elevator. I moved too quick.)
If you Olympic sprint and memorize the map you should have about 30 min to spare.
Also dying doesn't matter. You can die as many times as you want as long as you finish within 2 hours.