hard nut to crack

He saw the dilemma. . . . It was a hard nut to crack. He could see no way out of it.

The coast lad found the veteran Mike McTigue a hard nut to crack and judging from the look on the Californian's face when the final bell sounded, he was mighty happy that the fight was over.

The next day Archie, with Andrew Macpherson and Cluny Campbell, made their way through the woods until within sight of the castle. . . .

"It would be a hard nut to crack, Sir Archie," his lieutenant said. "Unless by famine, the place could scarce be taken."