Play the Torgue dlc until you can do the bar brawl repeatable. Begin mass slaughtering everything.
can u explain this more?
Just keep playing and you'll eventually find that there is a bar brawl side quest that eventually goes to Tier 3 Rematch, letting you re-do it over and over again. In this brawl, there are a lot of guaranteed Badass/high leveled enemies that spawn, giving out a great deal of EXP. So if you can mop them up very quickly, you can let the other person soak up all of that EXP.
Take a Level 50 Mechromancer to the Warrior, kill him, then use your Bot to constantly kill the Rakk in the area. You can leave your controller and go watch a movie. Make sure your Deathtrap has the skill that lets him stay longer for every kill. Each Rakk is over 200 xp and they are constantly spawning over and over infinitely. Your friend will level up very quickly this way.