This is earned during the first case Pray Tell
Continue into the case until you reach the section where you need to travel to Epping Forrest
As soon as the sequence starts Sherlock is hit by gunfire. There are two meters on screen, The top meter indicates the Hunters proximity to you. The bottom indicates your stamina.
Begin running straight away, hide behind the first large rock. You don't need to take every cover but make sure that you have enough stamina to get to the next. When resting wait until your bar is at least half full then move.
The hunters line of fire will appear on screen, as soon as it appears make a sharp left or right and you will avoid it. Stay to the left of the path this avoids annoying traps. If you hit a trap, mash A
Youre first choice is made for you. Head down the trail past a Boulder. If you go into the thorned bushes you will die, no matter the distance.
After another chase scene you make another decision. Choose Descent Rare Bushes
You will find a corpse, interact with the face or clothes. Unfortunatly you will not have time to assess him. Head down the path marked Swamp
The swamps are dangerous, you can only use Detective Mode to view the hidden path while in cover. So make sure you get a good look, before moving on. Essentially what you need to do is a lap around the swamp, so head up the hidden paths, using cover. When the line of fire appears quickly turn to avoid it. When you reach a flooded cabin, begin making your way back to the beginning. The Hunter will be out in the swamp and you can make a mad dash back.
You will come to another corpse and the achievement will unlock.
SoulyTheDouche commented:- 'why would you go back to the beginning when there is a path to get out of the swamps at the left of the cabin? achievement also unlocked without any problem'
Here is a video of this sequence which Stavia has recommended. Hope this helps