Step by Step guide on how to get Turtle Soup: Thanks to Chewy
1) This method only works in single player and you must have 2-4 players start the game. In the first level have three of them die, including the account(s) that you want to get the achievement with. Don't kill anyone with the account(s) you want to get the achievement.
2) Progress through the game until you get to Krang with one player. When Krang is near death bring back all the turtles and kill krang. Try to get the turtle/account(s) that is/are getting the achievement close to death before you kill krang.
3) Upon killing krang make sure that all the turtles are alive when shredder comes through the portal.
4) He will split into 6 clones of himself. Immediately allow the turtle/account(s) that is/are getting the achievement to be killed without being shrunk
5) Proceed to kill all of the clones with the other turtles that are not getting the achievement, this is done easiest with only one turtle alive playing as leonardo or donatello
6) When it is just you and the real shredder keep attacking him until his helmet comes off.
7) Once his helmet is off bring back all the turtles, his helmet must be off because when it is he will not use the shrink ray
8) Once you bring back all the turtles he will again split into 6 duplicates of himself but they will now all have their helmets off and not use the shrink ray
9) you may now kill the real shredder, but not with the account(s) who are getting the achievement.