Once you get to the endgame, you can start to re-run the main story missions on Hard or Challenging. Hard is actually not THAT hard, you just need a good squad. I started on WT1 and then progressed up to WT4.
The missions are:
Grand Washington Hotel
Jefferson Trade Center
American History Museum
Air & Space Museum
Space Administration HQ
Potomac Event Center
Federal Emergency Bunker
District Union Arena
Roosevelt Island <--- The last section is ROUGH! See my comments below for tips
PRO TIP: When at the end of Roosevelt make sure you use the Gatling guns to take the fuel dumps down to almost zero, but blow them up at the same time, otherwise you will have hoards more NPCs to deal with.
There is no DIRECT way to track progress, but you can do it INDIRECTLY.
- Go to Progress
- Hit RB to Commendations
- Open Combat
- You will see a bunch of commendations named "[Mission] Combat Merit" for getting 20 Headshots in a hard mission.
- Each mission has a "Mission Complete" entry, if this is ticked, then you completed the mission on Hard or Challenge.
EDIT: Go to commendations -> Service, and their should be a Service Merit for completing each mission on hard 10 times. If you've completed, it should say at least 1/10.
The last three are strongholds and a tad tougher. My recommendation is to run a turret and healing chem launcher. Have a mid- to long-range weapon (MMR, Rifle) and a shorter range weapon (SMG, AR, Shotty)
Just take your time, no face tanking and use cover effectively.