I've had a lot of trouble with the third Oxford mission in Righteous Fire - an escort mission that's extremely difficult because the enemies manage to take down the freighter you're supposed to escort in mere seconds! You can't even distract the enemies in order for them to attack you instead of the freighter.

So I accepted that mission, saved and quit the game and edited my save file.
I've attached a photo in which you can see which value you have to change to tell the game that the mission is completed.

Just edit that one symbol after the mission descriptor (in my case it was "s11md"), before the word "FORM".
The hex value of 80 (the "€" symbol), which you can see in the picture, indicates that the mission is still ongoing.
Change that value to BF (which will turn the "€" symbol into an upside-down question mark).
This sets the mission to "completed".

Thank goodness!
After about 30 tries, I began to HATE that one mission with a passion.
All the other missions after that are really managable.

Finally, the adventure can continue.