After you finish the game you unlock 2 secret characters. (I won't spoil who)
Instead of playing through the whole game again with one of them you can use a little trick to get this much much quicker. (this requires two controllers)
I loaded up my save, picked the "Loiter" option, and made sure to choose one of the secret characters. Now run all the way back to the final boss and use your second controller to select the character you beat the game with. (For me that was Kyoko)
Now the secret character is gonna be a much lower level than the character you beat the game with. (For example the secret character was level 5 & my Kyoko was level 23) So you just take the secret character and kind of put him in a corner (doesn't matter if he dies) and with player 2 controlling your beefed up character you beat the final boss and achievement unlocked!