After completing the first chapter, you can play community stories. Go to the link below, click on "Add to Story Log" and load it up in-game by pressing
, go to the "Quest" tab and in the bottom right corner select "See stories" to see it.
Track it and go to the mission location (close to the fast travel point on Kephallonia), trigger it and keep pressing
to repeat. Doing this will earn you infinite XP and money. The amount of XP and money depends on your level but it should take you between 1,5 hours or 2 hours to be level 50.
Credit obviously goes out to the author of the story; I didn't create it, only sharing it.