Another problem with this answer is that "fantastic fiction" spoken in normal English would most probably mean "really good" fiction, albeit that's a loose, colloquial use of the word 'fantastic' (but the most common). So you would really need to use the noun 'fantasy' because it's derivative adjective 'fantastic' has essentially lost its original sense in normal spoken English. 'Fantasy' would necessarily imply 'fiction' so you wouldn't have to specify that.
Agreed: if the need is felt to translate both words "fantasy fiction" would work, but "fantastic" is rarely used in its sense of "imaginative or fanciful, remote from reality", and certainly not in the phrase "fantastic fiction". Certainly searching the internet for "fantastic fiction" almost exclusively returns results listing recommended and very good fiction, not necessarily of the fantasy genre.
'Fiction' is «художня література», this term is much broader than «фантастика». I'm not sure there's an English word that directly corresponds to Ukrainian «фантастика».