Hi guys !
For this ending you must complete the game (any%) under 2 hours !
It's better to keep the 2 speedrun achievements (or at least this one) for your second run.You'll know the game and it'll be more manageable.
Here's my video for this achievement:
I used a big shortcut at 1:15:22.If you want to try this awesome trick (it's very easy to do,and you'll skip 25% of the game),check this video,it explain everything:
Have fun ! :D