This is posted in a few threads down, but anyway here ya go.
How to get Turtle Soup (info from xbox360achievements.org)
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.
Simply put, you need 2 controllers to get this achievement. It's
never been fixed, and it never will be. It's really annoying. I don't
have it, and I don't know if I ever will, even though I have a second
controller.
