Hyde: Hey Forman, I have an idea. Set up a wacky system of ropes and pullies, and when Casey gets here, drop an anvil on his head.
Fez: Because that's what Daffy does.
Eric: Yeah, I got that.
Hyde and Fez are referring to Daffy Duck from the Looney Tunes. Daffy tries to injure various other characters in a very indirect manner, such as dropping an anvil on their heads, and he always fails. Eric gets it because he knows he's using indirect methods to break up Donna and Casey, and it's not working.