jawbreaking (?.)

• Herculean
• abstruse
• arduous
• brutal
• complex
• critical
• delicate
• demanding
• difficile
• difficult
• exacting
• formidable
• hairy
• hard
• hard-earned
• hard-fought
• intricate
• knotted
• knotty
• laborious
• mean
• no picnic
• not easy
• operose
• polysyllabic
• rigorous
• rough
• rugged
• sesquipedal
• sesquipedalian
• set with thorns
• severe
• spiny
• steep
• strenuous
• thorny
• ticklish
• toilsome
• tough
• tricky
• uphill
• wicked