This achievement is story related and can't be missed.
*** Spoiler - click to reveal ***
After getting
talk to Dr. Fred.
He refuses to sign the contract, since he hasn't read it yet. Choose the last dialogue option that you save the world from purple tentacle yourself.
Dr. Fred will ask how you'll do it.
I chose that I'll declare him as insane and get him arrested. But since I need a note from a Doctor to declare purple tentacle insane, Dr. Fred will sign the "note".
With the contract signed, you unlocked the achievement.