As of the March 2018 update, all heroes are unlocked. Therefore, no more grinding. Just play a match of Heroes and Villains, switching characters when you die until you've used them all, then do so again after it switches sides. The tracker updates as soon as you use each hero, so as soon as you load in as the final one the achievement should pop.
Confirmed by Reborn Insanity. Thanks, man. :)