Here is an old fashioned idiom that might interest the OP. It means to attempt the impossible, or to place one difficulty on top of another.

To pile (or heap) Pelion on Ossa Add an extra difficulty to something which is already onerous.

‘But was it worth while to heap Pelion on Ossa, to shake the whole world, to create such a cataclysm of colour, merely to raise a smile?’ Oxford Living Dictionaries

Pelion

A wooded mountain in Greece, near the coast of SE Thessaly, rising to 1,548 m (5,079 ft). It was held in Greek mythology to be the home of the centaurs, and the giants were said to have piled Mounts Olympus and Ossa on its summit in their attempt to reach heaven and destroy the gods.

Knowing the idiom itself will set you apart from the masses.

This task is like piling Pelion on Ossa, as my professor of Classics was fond of saying.

If the OP is looking for the opposite of cakewalk (usually spelled as one word), then in British English there's the idiom wade through treacle

This task is like wading through thick treacle.

The British term treacle can be substituted with the AmEng molasses, without changing its meaning.