so I found that I had trouble with this one. because of enemies getting tougher the more days you stay and more enemies appearing I found myself fighting soldiers and a bear would often kill me from behind.
so I found the best way to get this achievement is to get a lantern and find a tomb. enter the tomb and put the lantern on the ground so both you and your partner don't freeze. now only go outside to kill animals for food. if you happen to be attacked by a bear run back to the tomb entrance and shoot it from there. the bear cannot enter the tomb and will take a few seconds to realize that before turning around and running away. I would recommend staying till day 8 morning just to be sure that its the full 7 days. this being said it will take a full hour and twenty minutes being as the days are ten minutes long each day.
extra tip: if you or your partner have unlimited poison arrows it makes things super easy especially against armoured enemies that show up when you are trying to leave