Phrase meaning "extreme difficulty"

Could someone please help? Thanks.

Although "difficulty amidst difficulty" is not incorrect, it is not a stock phrase, and to me, at least, does not sound like something a native-speaker would say. Usually, to describe a series of misfortunes (or good fortunes, for that matter, although that is less commonly referred to) one would say "one upon another," or "one piled upon another"--so that in this case, one might say "difficulties piled upon difficulties." Also, please note that that does not necessary mean extreme difficulty, just a continuous series of difficulties. A common locution which has grown up since the mid-20th century is "degree of difficulty," which has arisen from sports in which the athlete is rated on how difficult the judges perceive the attempt to have been. In that case, one would say "a high degree of difficulty." There are a whole series of stock phrases which describe someone in the midst of difficulties, such as "between a rock and a hard place;" "between the devil and the deep, blue sea;" and the nautical expression "the devil to pay and no pitch hot." (The explanation of that locution is obscure and long, so I'll leave that aside.) So, "the devil to pay," or "there'll be the devil to pay," has become a very common expression. It refers, of course, to the consequences of difficulties more than to the difficulties themselves.

Thanks, Setanta, for the detailed explanation.

Yes, a “series of difficulties” or “multiple difficulties” could be used.