His usual would not impress the Joker in the slightest. I honestly think the only way to scare the Joker would be to put him in a normal mundane life where no one notices or really responds to him. Any harm the Joker did to these people would not appear. Basically scare the Joker with sanity. This is of course contingent on Freddy (or his sanity) surviving long enough to figure this out.