Causing fear, dread, awe, or discouragement as a result of size, strength, or some other impressive feature; commanding respect; causing wonder or astonishment.
Difficult to defeat or overcome.
redoubtable
adj
Eliciting respect or fear; imposing; awe-inspiring.
How often have the words formidable and redoubtable occurred in a corpus of books? (source: Google Ngram Viewer )