This achievement doesn't need any special guide, since it's a time based, "play and wait" cheevo, but I thought I should say a few words.
WW2 introduces a new zombies mode, "The Tortured Path", in DLC 3.
In this new mode, there is a new, independent leveling system where you start in level
35 1-35 depending on your current level and must go to level 50. As your level increases, so does the overall damage you deal on zombies (120%, 150%, etc.)
All you have to do is play TTP and build up your level until you reach level 50.
There are specific things you can do to speed up your way to level 50:
- complete individual objectives (e.g. "kill all special zombies in time")
- "escape" the maps (just complete objectives and kill the bosses)
- complete the easter eggs for each map (complete all the ee steps, plus objectives and boss)
you can follow your current level in the soldier tab if you have TPP as the selected map.
as soon as you fill up level 49, you enter level 50, get the achievement, and enable "the hardened path", which is basically the game in hard mode. you will see an insta-kill when you start the map where you can enable hard mode.