The description says "Complete all Challenge Tombs in
one playthrough"
After some days and reports from the users, I think we have the riddle solved :)
One playthrough means dont start a new game, thats all. You can get this achievement after the story, on multiple savegames, all is fine when you play and solve the 9 tombs in your game without starting a new game. (thanks Tecstar70 in the comments)
Want to go totally safe but it is not necessary, then solve
all 9 tombs in the game before you finish the story. At the last base camp in the Lost City area you get a warning from the Game "Point of no Return...." Dont go forward here and solve as first the 9 tombs.
Here are my 9 walkthrough videos for all tombs.
The videos are including all collectibles in the tombs and are straight to point. (There is no testing or trying, real walkthroughs)
The first tomb vid is incl. the entire area collectibles in Glacial Cavern.
Ice Ship Tomb (Glacial Cavern)
Ancient Cistern (Soviet Installation)
Voice of God (Soviet Installation)
The Red Mine (Soviet Installation)
Baths of Kitezh (Geothermal Valley)
House of the Afflicted (Geothermal Valley)
The Pit of Judgment (Geothermal Valley)
Catacomb of Sacred Waters (Geothermal Valley)
Chamber of Exorcism (The Lost City)