It's a bit tougher, which is why I usually didn't bother. I'd start by accusing them of robbing a bank, and when they would inevitably say they didn't, I'd point out that my made-up testimony alone is just as verifiable as their legitimate one alone. Then I'd explain how being able to convince a jury of people - who weren't present at the time of the crime - that a specific person is guilty is a good way to make sure there's actually enough verifiable evidence to ensure it happened, and isn't just a setup by someone lying to get them in trouble.

If the crime was so blatant that people obviously saw it happen, there would ideally be enough physical evidence that the crime happened, and if there isn't, then it would still be preferable to let a criminal walk free rather than make an innocent person go to jail because they couldn't prove they didn't do what someone else said they did.

The tough thing is that some people have too large of an ego, and will just ignore that entire point, thinking that the people they "know" to be criminals should just be locked up because "it's obvious," and then we get to the situation happening now.