There's always a downside to every approach though. Some people get flustered if they have to say "I don't know" too often, so instead of bouncing from tough to

There's always a downside to every approach though. Some people get flustered if they have to say "I don't know" too often, so instead of bouncing from tough to easy, I'd probably prefer to progress from easy to tough. People will feel confident until they hit their limits. Their limits are what you are looking for (presumably), not their ability to think clearly when they are scared they just blew the interview on a question you didn't really expect them to answer.