Use the "Infinite XP" Speech trick to easily get the "Silver Tongued Devil" achievement.

Go to Little Lamplight at any point after you exit Vault 101. Get inside via Speech Challenge or rescuing other children from Paradise Falls. Go back to the "Great Chamber", and find Knock Knock. Ask her how she got her name. Have her tell you a joke. She only has one joke, and with a decent Intelligence, you can interrupt it for a better ending. Then, there is a Speech Challenge to convince her that it was a funny joke. Continuously cycle through the joke and Speech Challenge repeatedly. Because she is a child, the Speech challenge is not too difficult.