This should come to you as you progress to the final level of your second playthrough. XP can be earned by killing enemies, completing quests, and completing challenges (Challenges can be found by pressing
under the quests tab). Re-run the last area of the game on your second playthrough if needed.
Power-leveling can be used to get an early boost this and will definitely help as you progress through the game. To power level, it is required that you have at least 2 players, one who is just starting the game and the other who has completed at least Playthrough 1. The higher level character must teleport to the last level of the game in Playthrough 1 or 2 and rush through and kill all of the enemies. This will allow very large XP boosts to go to the lower-level player. It is possible to get to level 20 during one run and this is very helpful when starting out.