This is an odd one, first you need to complete the second mission in chapter 2 to unlock the arena. Next make sure you have a lower level character (that needs leveling) as your lead and speak to the man on the right in the arena. Save the game at him and then leave for 2 - 24hrs. When you reload the save you'll have earned EXP on the character left there and the achievement will pop.
You can replay missions in the meantime while the 2 hour minimum training is going on.
And for note approx 2hrs of 'training' while i re-ran a couple of the missions netted 1350EXP for a level 11 character. Very useful to level your lesser used characters while you sleep or work ^^
Edit: This 'can' also be used to power level characters by going offline and manually changing the clock forward a day or 2. Wouldn't recommend doing this unless you forget about a character for a while and find the severely under-levelled