Not really a solution but a note: I did a run with a friend, and it counted for 2 of the toads in one playthrough.
If you have 2 friends to play with you, you can knock it out in one go.
Important note: My friend was signed in also, but he did NOT get any achievements from the game. We finished a playthrough but he didn't get "Finish the Fight," only I did. So whomever is the active account, gets the achievement.
Edit: From Borgenator (this guy is awesome)
If you can't get three people together, and you have an extra controller, you can save just before beating the final boss and then sign the extra controller into an alternate account, and have them jump in the game.
Beat the boss and you'll get two toads. Reload the save then have the extra controller be the last toad you need and beat the game again. You can still get this in one go and as little of the last boss as you would like to play twice.