Synonyms and related words :
Herculean, abstruse, alarming, appalling, arduous, astonishing, astounding, awe-inspiring, awesome, awful, brutal, burdensome, challenging, complex, conspicuous, critical, daunting, delicate, demanding, difficile, difficult, dire, direful, dread, dreaded, dreadful, earnest, effortful, egregious, exacting, exceptional, extraordinary, fabulous, fantastic, fatal, fateful, fell, freaky, frightening, frightful, ghastly, ghoulish, grave, grim, grisly, gruesome, hairy, hard, hard-earned, hard-fought, heavy, hideous, horrendous, horrible, horrid, horrific, horrifying, imposing, impressive, incredible, indomitable, intimidating, intricate, jawbreaking, knotted, knotty, labored, laborious, macabre, marked, marvelous, mean, menacing, mighty, mind-blowing, mind-boggling, morbid, no picnic, not easy, notable, noteworthy, noticeable, of mark, onerous, operose, outstanding, overwhelming, petrifying, portentous, powerful, prodigious, redoubtable, remarkable, rigorous, rough, rude, rugged, schrecklich, serious, set with thorns, severe, shocking, signal, sober, solemn, spiny, staggering, steep, strenuous, striking, superior, terrible, terrific, terrifying, thorny, ticklish, toilsome, tough, tremendous, tricky, unbelievable, uncommon, uphill, weighty, wicked, wonderful