piece of cake

[blouen]

* What's the antonym of this idiom?

[susiedqq]

Idioms usually don't have antonyms. They are unique and have their own special use in any conversation.

"That test was a piece of cake."

[blouen]

I mean. Do you know an idiom which means, something is not easy or it is difficult?

[riverkid]

I mean. Do you know an idiom which means, something is not easy or it is difficult?

be a [real] uphill slog/be a slog

be tough as hell

be a task of monumental proportions

be easier said than done

be a pain in the neck

be no easy pickin's

not an easy task/job/decision

not the easiest ... to do

be a tall order

a daunting task

be murder to do sth

backbreaking

There are more, B, depending on the area of meaning.