Zhè ge xiàngmù shì kuài yìnggǔtou, děi mànmàn kěn.
This project is a hard nut to crack — we'll have to chew at it slowly.
Tips
usage
Works for people and tasks. For people it's admiring — unbreakable spirit. For tasks it's the one you 啃 (gnaw at) because it won't yield easily. Often used with 块 as a measure word for the task sense.
