This game, and therefore all of its achievements, have been discontinued.
Once you've gotten a hero to level 60, you can go to the Heroes menu and press Y to prestige them - you'll get the warning that it'll reset them to level 1, and any equipment they had will be removed unless it works with a level 1 character. It'll also warn you that it takes 25% more experience to re-level the hero.
You're gonna do that, twice.
For me, I used Dr. Strange. My plan to quickly farm experience was to rotate through the first few treasure rooms. From Hells Kitchen - South, I went to the two treasure rooms following the linear path of the level (Bajillion and the Market). Then I fast-travelled to Hell's Kitchen - North and went to the Rooftops treasure room along the main path at the beginning. After that, I fast-travelled to The Abandoned Subway and hit the early treasure room (Forgotten Station) - once I was over level 10 I'd use my travel power to go beat Shocker at the end of that level. Finally, I'd warp to the docks at the start of Chapter two and do to the first two treasure rooms (the first one is north of where The Hood's weird power reading is, and then you go east past where the first alert is and turn right to go down to the Hammerhead Hideout. Once all of those were done - it took about twelve minutes as a casual pace - I'd go back to a Headquarters and switch heroes, but then switch right back to reset all the treasure rooms. It goes much faster if you add experience boosts, and if you do the Heroic and/or Super Heroic Trials.
Once you prestige and drop back to level 1 for the second time, the achievement will pop. From there, it's up to you if you want to level them up ever again.
Also of note, if you decide pick a hero to do this with early on, they'd be a great choice to equip with a legendary item for the grindy achievement for fully-levelling one.