tough nut informal
hard nut informal

"This is really one of the tough nuts to crack," he says.

His work, and this show in particular, is a tough nut to crack.

He'd be a tough nut to crack if he disagreed with you.

Julia might be the toughest nut of the three, but I'm working on that.

They've been working on that group for quite a while, and it's a tough nut.

They look a fit, athletic team and are going to be a tough nut to crack.

I've seen London before, and he's a bit of a tough nut.

Agreed, there are tough nuts at the work camp, but do you know how to handle them?

Human beings are tougher nuts, can doubt anything, even the evidence of their own eyes.

They're a very tough nut to crack when they've got their noses in front.