Teenage Mutant Ninja Turtles: Shredder’s Revenge Achievement No Need for Mutagen!

You dont need any preparation for this method, since we are going to pick stage 1 in history with any difficult for get the achievement.
The basic idea is to play stage 1 in history, but at the start of the stage you should use a 2nd controller for leave a player in stand-by, this way the game is going to give you more ammount of enemies for get exp, now just play the stage from start until the boss, and when you are about to finish the boss, make the 2nd player you have in stand-by to join the game (be sure that your 2nd player have a character that is not at max level), this way you are going to be getting a total of 150 exp and the 2nd player a free 15exp. After the end just rinse and repeat, you need about 13 and half runs in stage 1 for reach max level (you need a total of 2000 exp for max level).
This works the same for all the characters, just try to beat the stage 1 the fastest you can, rinse and repeat.

Im using this method in the easiest difficult, and is taking more or less 1 hour for reach max level with a single character and a 2nd player should be getting some exp too. It is still grindy but way faster than just playing the campaign normal.
When you hit max level or 2000 exp, you can just exit the stage, since the game autosave the progress, you can try changing the stage too if you feel burned, but i definitelly recommend stage 1 since is the easiest and fastest.

If you are carefull and pick April, you can also get the achievement: Cowabunga it is, by performing a 250 hits combo, i play without giving too much attention to the combo and i manage to get it like 3 times at least.

Full playthrought using this method and leveling April from 0 to max level in 1 hour here: