User blog comment:EmpyreanSmoke/Supernatural Q&A/@comment-95.112.157.153-20150812172827/@comment-24834479-20150815192333

If Death were still around, he would probably be able to kill God, since he himself is Death.

Castiel was unstable mostly because of the Levithan, so if they weren't there, I think Castiel would have been able to manage it.