Getting them to answer the questions is like pulling nails

This is a mix of “like pulling teeth” (difficult) and “tough as nails” (strong and determined), both describing tough situations.  I think part of the mix is the visual of actually extracting or pulling a nail.  A big thanks to Jack de Golia, who sent me a list of malaphors and malaprops that his wife has uttered over the years.  This one is a particular favorite of mine.