He has mastered a 'formidable' amount of material.
The mountains were a 'formidable' barrier.
a 'formidable' enemy/opponent/weapon
(..) Irony bounds. Nevermind... It's safe. In London. Where my colleagues are making good use of it. The most formidable criminal mind in Europe, has just had all his money stolen, By perhaps the most inept... inspector in the history of Scotland Yard. Any chance of a cup of tea. How much more is there? (..)2011 Sherlock Holmes: A Game of Shadows