The best way to farm Mist Echoes and get the most out of Nilfheim Chests is to complete the maze with as many chests opened as you can find. (How to unlock Nilfheim)
Nilfheim is a new realm that becomes available after you found the 4 language cipher in different pink chests around the map. This arena like location is covered with deadly fog that will take Kratos down if he stays in it too long. The red bar on top of the screen indicates the time you have left.
How to Farm Mist Echoes
Start your run from the center and remember the symbols on the nornir chest at the beginning. There are 3 statues in the maze that you need to activate with your axe. When you complete the run you can open it and collect the biggest mist echo reward.
Make sure you start each run in the same direction. This way you get to know the maze better and better each run and won’t get confused as much.
The trick to getting 10000-13000 mist echos is to open most chests in the nilfheim maze. Everytime you open one chest you increase the amount you get from the next chest. Keep in mind that you lose all your progress and mist echoes when you die.