arduous (adj.)

• abrupt
• abstruse
• backbreaking
• brutal
• burdensome
• complex
• critical
• crushing
• delicate
• demanding
• difficult
• effortful
• energetic
• exacting
• exhausting
• forced
• formidable
• grueling
• hairy
• hard
• heavy
• hefty
• intricate
• killing
• knotted
• knotty
• labored
• laborious
• mean
• onerous
• operose
• oppressive
• painful
• precipitate
• precipitous
• punishing
• rigorous
• rough
• rugged
• severe
• sheer
• spiny
• steep
• strained
• strenuous
• taxing
• thorny
• ticklish
• tiring
• toilsome
• tough
• tricksy
• tricky
• troublesome
• trying
• uphill
• vigorous
• wearisome
• wicked