1. in an extremely difficult or challenging way
2. in a cruel or evil manner

Examples

• The math problem was fiendishly difficult.
• She smiled fiendishly as she revealed her plan.
• The puzzle was fiendishly clever in its design.
• He laughed fiendishly at his opponent’s mistake.
• The test questions were fiendishly tricky.
• The villain grinned fiendishly before disappearing.