as different as chalk and cheese

a difficult phase

He’s a difficult one to pin down

He is a difficult man to deal with.

It is a difficult task, but that is all the more reason to work hard at it

It is difficult.

a difficult task to undertake

to diffuse widely the proposals of the Working Group

to dig

to dig a little deeper

to dig around

to dig deep

to dig in

to dig in one’s heels

to dig into

to dig off

to dig oneself in

to dig one’s fingers/nails into something

to dig one’s own grave

to dig one’s way

to dig out

to dig something up/dig up something

to dig something

dig in the ribs

Dig in!