To build a high Badass Rank and earn all the perks associated with it, make a pair of new characters in co-op mode and have player 1 beat player 2 in 50 duels. This offers 166 Badass Ranks. Save and quit, then start another new game, creating another new character for player 1 (leave player 2's character alone). Fight another 50 duels with player 1 winning them all for 166 more Badass Ranks. Repeat as desired, and delete the pile of unwanted characters when you're done. If you set the controller configuration to something where you can just keep tapping the melee-attack button without ruining your aim at the other player, you can end a duel-run like this in roughly 10 minutes. So in one hour you can build up 996 Badass Ranks.