I've said this before, but Oliver shows it very dramatically. The way to beat Trump is by using his own words against him. Challenge his business acumen. Find all those dissatisfied customers and whatever legal documentation to show just how much he sues people, and how often he's been sued. Trump University will be an anchor around his neck.
The tax returns could be a huge key in defeating Trump. I suspect he doesn't want them released is because he's lied and inflated his personal worth to such an extent he can't talk his way out of it. I suspect Trump Inc. supports a lavish lifestyle that even the most jaded publicly traded CEO's couldn't stomach. I don't think his ego can take the truth that minus his inheritance he's probably at a break even point.