Definition of formidable:
shocking, hair-raising, stiff, exacting, terrible, frightening, awesome, imposing, herculean, challenging, fearsome, taxing, rigorous, testing, overwhelming, Augean, fearful, horrifying, redoubted, impressive, redoubtable, alarming, weighty, grueling, impregnable, killer, tremendous, grand, murderous, tall, terrifying, dire, unnerving, sweaty, pick-and-shovel, effortful, horrendous, forbidding, appalling, exigent, moiling, dread, fear, heavy, dangerous, dreadful, ghastly, backbreaking, hellacious, intimidating, frightful, direful, spine-chilling, scary, horrible.
difficult (part of speech: adjective)
- powerful (part of speech: adjective)
- strong (part of speech: adjective)
- invulnerable (part of speech: adjective)
- "Monks, Popes, and their Political Intrigues", John Alberger.
- "The Legacy of Cain", Wilkie Collins.
- "The Golden Silence", C. N. Williamson and A. M. Williamson.