formidable

Synonyms

arduous, Augean, backbreaking, challenging, demanding, difficult, effortful, exacting, grueling (or gruelling), hard, heavy, herculean, killer, laborious, moiling, murderous, rigorous, rough, rugged, severe, stiff, strenuous, sweaty, tall, testing, toilsome, tough, uphill;

Antonyms

cheap, easy, effortless, facile, light, mindless, simple, soft, undemanding;

Near Antonyms

achievable, clear, doable, elementary, manageable, uncomplicated;

comforting, gentle, painless, relaxed, smooth, soothing;

accessible, friendly, idiotproof, user-friendly;

Related Words

abstract, abstruse, complex, complicated, elusive, hairy, insoluble, intricate, involved, knotty, opaque, problematic (also problematical), recondite, serious, spiny, stubborn, thorny, ticklish, tricky;

bruising, burdensome, exhausting, labored, onerous, oppressive, stressful, taxing, tight, trying;

annoying, bothersome, distressing, irksome, troublesome, vexatious;

grievous, grim, strict, stringent;

brutal, cruel, inhuman, painful;