I'm finally done! It indeed did unlock after completing the game. Just took a few hours, luckily the four tombs were unlocked right of the bat!
Here is a workaround to get Legendary Freelancer challenge. (post patch 1.1)
1. Go to javelins in Forge
2. Create a new loadout and when asked to reset or keep, choose reset.
3. Now just put on gear until it reaches the desired level. (Common, Epic, Masterwork, etc)
4. Choose a contract and change difficulty if needed.
5. After completing the expedition, you'll notice the checkbox is marked off that you'll need.
6. Rinse and repeat for each level.
7. This way, you do not have to repeat the whole campaign again on a new javelin.