Synonyms for Formidable:

adj.

amazing (adjective)
awesome.
ambitious (adjective)
severe, impressive, strenuous.
difficult (adjective)
cumbersome, burdensome, demanding, fussy, complex, trying, thorny, severe, tight, knotty, complicated, uphill, hard, finicky, wearing, rugged, adverse, harsh, stressful, difficult, contrary, tricky, onerous, troublesome, grinding, arduous, cantankerous, wearying, bothersome, ticklish, wearisome, laborious, tough, oppressive, strenuous, rough, effortful, toilsome.
difficult, overwhelming (adjective)
rough, mighty, tough, hard, impressive, awesome, onerous, toilsome, laborious, effortful, strenuous, uphill, powerful, arduous.
fearsome (adjective)
fearsome.
horrible, terrifying (adjective)
appalling, fearful, frightful, dreadful, impregnable, dire, redoubtable, imposing.
invulnerable (adjective)
impenetrable, indestructible, impervious, unbeatable, invulnerable, immune, invincible.
powerful (adjective)
commanding, high-powered, dominant, supreme, influential, autocratic, important.
strong (adjective)
mettlesome, muscular, mighty, brawny, robust, stout, husky, energetic, powerful, firm, stable, virile, potent, vigorous, vital, tough, strong, sturdy, sinewy.

n.

forbidding, fearful, impregnable. overwhelming. grand, awesome, imposing. scary, appalling, dire, fear, ghastly, dreadful, fearsome, frightful, direful. backbreaking, exigent, heavy, exacting, taxing, effortful, weighty, rigorous. formidable (noun)
impressive, redoubtable, unnerving, alarming.

Other synonyms:

direful, fearsome, scary, awesome, backbreaking, effortful. fearful. exigent, rigorous, weighty, appalling, dire, ghastly. exacting, frightful, taxing, dreadful. heavy. Other relevant words:
alarming, weighty, appalling, dire, overwhelming, exacting, unnerving, taxing, influential, backbreaking, imposing, rigorous, forbidding, important, redoubtable, fearsome, scary, dreadful, awesome, direful, fearful, frightful, impressive, ghastly, heavy, effortful, impregnable, dominant, exigent.

Usage examples for formidable

  1. He began to breathe again, to regain his footing, when other attacks equally formidable presented themselves. – En Route by J.-K. (Joris-Karl) Huysmans
  2. They investigated it with solemn gravity and found it very formidable from Spain the panic spread through the whole of Europe. – Modern Magic by Maximilian Schele de Vere
  3. At present, however, it still seemed a formidable task for a team that was to pull me over thirty- three miles more. – Over Prairie Trails by Frederick Philip Grove