formidable (adj.)

• abstruse
• alarming
• appalling
• arduous
• astonishing
• astounding
• awe-inspiring
• awesome
• awful
• brutal
• burdensome
• challenging
• complex
• conspicuous
• critical
• daunting
• delicate
• demanding
• 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-fought
• heavy
• hideous
• horrendous
• horrible
• horrid
• horrific
• horrifying
• imposing
• impressive
• incredible
• indomitable
• intimidating
• intricate
• knotted
• knotty
• labored
• laborious
• macabre
• marked
• marvelous
• mean
• menacing
• mighty
• mind-blowing
• mind-boggling
• morbid
• notable
• noteworthy
• noticeable
• onerous
• operose
• outstanding
• overwhelming
• petrifying
• portentous
• powerful
• prodigious
• redoubtable
• remarkable
• rigorous
• rough
• rude
• rugged
• serious
• 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