tough nut informal
hard nut informal

Apart from anything else Bill was a right hard nut.

But as with many a hard nut, it's probably worth the effort.

She was going to be a hard nut to crack.

South London's hard nuts had failed to turn up, though.

The political question, however, could be a harder nut to crack.

That was not ideal, but this had been one hard nut to crack.

"At this stage we cannot rule anything out but there are going to be some really hard nuts to crack," an officer said.

Bxe6 fxe6 his position remains a hard nut to crack.

Had they not bit again and again on that hard nut these past months?

A hard nut to crack, admittedly, but there won't be another chance for quite a few years.