An easy way to get this for half the work is to have a second player play another DLC character. Once the costumes are all unlocked, everyone in the group gets the achievement. My g/f & I used this method to unlock the Hero & Villain DLC costumes and it worked fine. We split our group into 2 Hero DLC characters & 2 Villain DLC characters.
Of course, if one of you already has the DLC costume achievement, then this method is only beneficial to one person.