This achievement should take roughly 15 to 20 hours of simulated battles in the Unfair challenge.
This achievement requires reaching level 80 with anyone of your fighters. I choose Horatio and made him my hero and full blown tank. I then selected zombies, electrobot, kobolds, pixie and vampire. I headed to the pit to do the Unfair challenge. Inside the match I turned on Auto Battle and let the computer fight for me. After each wave is done you can tap A button on controller to start next wave. You can continue to keep hitting A button after each match and it will automatically go into next fight still on Auto Pilot. You can essentially lay in bed while watching Netflix and Tap A button on your controller and let the computer simulate each fight up till level 80.