Synonyms of formidable in English:

intimidating, forbidding, redoubtable, daunting, alarming, frightening, terrifying, petrifying, horrifying, chilling, disturbing, disquieting, dreadful, brooding, awesome, fearsome, ominous, foreboding, sinister, menacing, mean-looking, threatening, dangerous

informal spooky, scary, creepy

archaic direful, bodeful

onerous, arduous, taxing, difficult, hard, heavy, laborious, burdensome, strenuous, vigorous, back-breaking, stiff, uphill, relentless, Herculean, monumental, colossal

demanding, trying, tough, challenging, exacting, overwhelming

exhausting, wearying, fatiguing, tiring, punishing, gruelling, grinding

informal killing, no picnic

British informal knackering

rare toilsome, exigent

capable, able, proficient, adept, adroit, accomplished

impressive, strong, powerful, mighty, terrific, tremendous, great, redoubtable, indomitable, invincible

seasoned, skilful, skilled, gifted, talented, masterly, virtuoso, expert, knowledgeable, qualified, trained

efficient, good, excellent, brilliant, outstanding, first-class, first-rate

informal mean, wicked, deadly, nifty, crack, ace, stellar, wizard, magic

North American informal crackerjack, badass

vulgar slang shit-hot

archaic, humorous compleat

rare habile