In order to get this you'll have to get at least one of the four characters to level 45. It sounds like a lot but its quite easy to rank up and you should be around 38 to 40 by the time you finish the game. So, you shouldn't have to grind on it too much. You get experience from destroying enemies and doing primary and secondary objectives. Remember, you can only gain levels by playing on normal or hardcore.
Note that some people are having problems with this, as their characters are resetting themselves once they get around level 35 to 40. I've got it and know of other people getting it so it's not impossible but it is more than likely glitched. You may want to wait for a patch/update or can attempt it at your own risk.