Ok, so there is a XP exploit in this game. If you make a manual save right before you complete an objective, you can reload that save and you will gain xp over and over.
I've uploaded a video guide from the DLC
Target Führer where I show a very quick way to infiltrate the factory and provoke Hittler, making him rush toward the escape boat and after killing him, awarding you an instant 6000xp
IF you make a manual save right after he embarks the boat (at 06:00 in my video).
Rinse and repeat until you get the desired level or until you get bored
Happy achievement hunting and please state the reason if you give this solution a negative vote
BTW, I apologise for the "brakes" at 06:45 in my video but my gamer tag and video description are obscuring the level wheel on the top right of the screen. Cause I wanted to show that you have to wait for the 3 challenge rewards to appear on the screen to be able to get the 6000xp. If you pause and load the game right after you get the cutscene, you will be rewarded only the xp that managed to appear on screen. You can see what I mean by pausing the game after each xp reward and checking the level wheel in the top right of the screen
Edit: thanks to
FuzzyRabbidDogg for pointing out that you have to complete the mission first before you can replay it to do any challenges
Edit: The next morning when I did some more xp grinding, I got 8000xp, adding 2000xp to the initial exploit by completing the challenge "quick game of squash" even though I didn't actually crush Hittler under anything. I've added the video of one of my 8000xp shots below as a guide to where I'm shooting