You will likely be at about level 36-38 after completing the game once, doing pretty much everything possible. From this point, if you can find someone who is already level 50, join them and grind out the remainder to make it go a little faster. Find a highly populated area of zombies (the more special ones the better) and grind there. Using your Fury skill to kill enemies awards bonus experience, especially if you kill enough to get your multiplier up a few ranks.
The best way to get to level 50 is probably abusing the repeatable quest "Dreamtime" as shown below. To duplicate a weapon, hold to ready it for throwing, then hold at same time, and press . If you're using Logan and your "Boomerang" skills pops, it won't dupe. Otherwise you'll now have two of the same weapon on the floor. So in essence:
Dupe a ton of your best, high-selling weapon.
Sell them all and buy as much Oleander as possible.
Turn in x5 Oleander for 1000xp in the repeatable "Dreamtime" quest in the Jungle.
Another excellent method is abusing the passive Thug inside Harlan's Hut during the "Make yourself at home" quest in Act 1. Once you beat the game, start a New Game+ and complete the quests Passport to Life, Lazarus Rising and Ashes to Ashes, then drive to the location in the video. Save Harlan and begin the quest mentioned previously. Make sure you DO NOT end the quest, and try not to kill the thug. When its health is low, reload your last checkpoint to reset the Thug. Follow the steps below:
Accept the quest from Harland and break into the house.
Break the Thug's arms (may need to use a stronger weapon).
Use a pair of BROKEN brass knuckles to punch at the broken arms.