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.