Possible Clues:

• It's difficult to get high on demand
• 'I want a skyscraper built by tomorrow' - you'll be lucky!
• Difficult task, request for 12oz coffee in Starbucks?
• Demanding task for fraternity of high standing
• Hard-to-meet requirement
• Formidable task
• Hard thing to fill
• Tough request
• Tough thing to fill
• Lofty command - that's a difficult task