You can set up the remote debugging. In the Run/Debug Configurations:
Then when starting your solr with the default jetty (this one is in multicore mode), pass in:
java -agentlib:jdwp=transport=dt_socket,server=y,suspend=n,address=8000 -Dsolr.solr.home=multicore -jar start.jar
After solr has started hit Run -> Debug button and choose the debug config. The IntelliJ IDEA attaches to the Solr java process and all the debugging goodies should be available to you.