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)
fussy, thorny, cumbersome, contrary, strenuous, arduous, laborious, demanding, knotty, rough, uphill, ticklish, tricky, finicky, harsh, grinding, troublesome, difficult, wearing, tough, stressful, wearying, complicated, complex, cantankerous, tight, severe, burdensome, wearisome, hard, adverse, toilsome, onerous, rugged, oppressive, trying, bothersome
powerful (part of speech: adjective)
strong (part of speech: adjective)
invulnerable (part of speech: adjective)
- The return of his formidable army alarmed the fears of the pope, who appears to have equally dreaded the success of his arms abroad and of his presence at home. - "Monks, Popes, and their Political Intrigues", John Alberger.
- My memory of what passed between us is, in some degree, disturbed by the formidable interruption which brought our talk to an end. - "The Legacy of Cain", Wilkie Collins.
- She recognized him as she looked down, and started back with an impulse to escape, he seemed so near and so formidable. - "The Golden Silence", C. N. Williamson and A. M. Williamson.