This can be grinded rather quickly by only accepting favors such as "Mathew wants you to beat up Colin"
Once you have at least 2 "beat up" favors make sure to sleep in the evening to save your game.
When you wake up it'll be time for morning roll call and at morning roll call you'll have everyone lined up and ready to be beatdown.
When you've either finished your favors or you die just pause and reload.
You'll still have the favors active in your journal.
Below is a video showing how to grind it out.
You'll want to earn the following achievement first as you'll need the health/speed to get away when the guards chase you around.
Ideally you would want to do this before you escape the prison as better armor/weapons will make it easier....but they aren't neccessary. I do not recommend using just your fists though.
Some people only took one hit but some took two. I think this has to do with their own personal stats.
The more favors the quckier the grind will be. 2 is good but 3-4 is best.