The real Purple Tentacle won't let me turn off the Sludge-O-Matic!
Hint 1 of 7:
I think that it's time to give Purple Tentacle a taste of his own medicine.
Hint 2 of 7:
Or of his own technology in this case.
Hint 3 of 7:
Try talking to him so that he fires the gun on himself.
Hint 4 of 7:
The shrink ray only works if it's shot in the middle of a person's forehead.
Hint 5 of 7:
Dr. Fred has a reflective forehead. Maybe that could help you.
Hint 6 of 7:
Tell Purple Tentacle to shoot Dr. Fred.
Hint 7 of 7:
The dialogue path follows: