Point out all the right wing things he's done, that Hillary Clinton has opposed. Then, Hillary will win the Dem nomination and Trump will steamroll her.

Bernie Sanders voted against immigration reform in 2007 and wants less legal immigration:

http://www.nytimes.com/politics/first-draft/2015/10/19/bernie-sanders-cant-escape-questions-about-2007-vote-on-immigration-overhaul/

Bernie Sanders voted 5 times against the Brady Bill:

http://www.politifact.com/truth-o-meter/statements/2015/oct/13/hillary-clinton/hillary-clinton-bernie-sanders-voted-against-brady/

Bernie thinks open borders is a capitalist conspiracy to lower wages of Americans

https://www.youtube.com/watch?v=vf-k6qOfXz0

All this stuff can make him seem like a Stalinist. He went to the USSR for his honeymoon too, which blocked off immigration from all of Eastern Europe.