What is the comparative of 'difficulty'?

"Difficulty" is a noun and, as such, does not have a comparative form. The comparative form of the adjective difficult is more difficult.

The use of the comparative adjective 'greater' makes the noun 'difficulty' comparative. The phrase proceeds from 'difficulty', to the comparative 'greater difficulty' and the superlative 'greatest difficulty'. This compares with a similar progression for the adjective 'difficult'. The simple 'difficult' becomes the comparative 'more difficult' and the superlative 'most difficult'.