This is mostly self explanatory but I'll provide a couple of pointers for obtaining this.
This can be started through the Endurance Mode, selecting Co-op. This requires a Co-op partner.
Any difficulty is allowed, I suggest Adventurer, the easiest.
Namely, one question I had was, you have to COMPLETE 7 days, ie reach the 8th day before extracting.
Helpful cards include Tomb Raider (for the codex skills which allow you to see traps in the tombs), either infinite fire or poison arrows (I had equal success with both).
Look for and pick up a lantern (usually near a settlement or basecamp) and keep this with you for guaranteed warmth at all times. Leave this at the entrance of a tomb or you will not be able to get it back up in most cases.
Pointer on finding Tombs/Crypts: always move AWAY from other landmarks you have found, we spread out from each other a bit and pretty much just ran in parallel in the same direction away from the bobcat until we found another Tomb.
In the Tombs, use Tomb Raider sense (or whatever it's called, press RS) frequently, this will help you avoid the traps (they show up red if you have the codex skills). All traps can be triggered ahead of time by arrows, this is recommended, especially the floor spike traps which seem to have a larger trigger stamp when not yet triggered.
Once you have obtained all of your artifacts I recommend that you pick a tomb and stay in it with your lantern, venturing out only to hunt for food. You need to hunt ~2x per day in order to not starve, otherwise, you can hang out, care free at the entrance. (Credit to Warrz98 for his solution to Team Survival).
This can be stacked with single player challenges such as Great Haul (10 days, 10 artifacts), the 7 day challenges (Bonding Time, etc.) or Adventure Besties (5 medium or large artifacts).