To those who support him, Trump represents a refreshing alternative to the status quo. These individuals appear less concerned with the substance of Trump’s campaign (or lack thereof), and more attracted to the anti-establishment sentiments surrounding it.
If any candidate has any hope of presenting a realistic challenge to him, they need to begin to understand this. Attacking Trump on policy is obviously not working. If you want to take him out, address the anger and fear driving his campaign.
Simply put, defeating Trump is more a matter of understanding his supporters than understanding him.