How did I progress from level 53 (after all story missions) to 60? Well, after acquiring the level 52 shock Rosie, I specced into Tombstone, Unchained and High Noon, put on a High Rollin’ Crapshooter with +6 Tombstone and +5 Unchained, set myself up behind the sera guardian of the guardian hunter mission as to minimize lag and time spent per kill, popped Showdown, pressed the left mouse button, pressed Shift+Tab twice to open and close the steam overlay, which locked the left mouse button and continued to shoot my rosie, and then Alt+Tabbed out of the game.
Half an hour later I was level 60 and still killed 6-7 sera guardians per Rosie clip (based on the amount of Impatience stacks gained), and made a huge mistake by having picked up Unforgiven whose explosion at the end of Showdown crashed my game.
Then I did it again and again because it crashed a second time after looking at the horrendous amount of bodies.
(As you can see, I levelled to 60 without ending showdown even once.)
Now am level 60 without too much hassle. Wasn’t fun, but at least it was easy. Also, level 49 maggies work wonders with Tombstone and Hot Lead against level 60 enemies, save for bosses of course.