You can first get a good understanding of 하기 and its usage. 하기 is the noun form of the verb 하다(do), what "doing" is to "do". The noun form can also create a long phrase: 이렇게 어려운 일을 빨리 하기 (Doing such a difficult task quickly). - 이 일은 빨리 하기 어렵다: It's difficult to do this task quickly. We can think of it like Subject(이일은) + Predicate Sub-clause(빨리 하기 어렵다), and 하기 is the subject in the clause. (A different grammatical interpretation is also possible) That is, 하기(가) 어렵다 = Doing (it) + is difficult, where "doing (하기)" is the subject. So 하기를 어렵다 is ungrammatical and never used because 를 is strictly an object marker - there's no exception. Both 하기는 어렵다 and 하기가 어렵다 are good since 는 and 가 mark the subject (or topic) of the sentence. 하기 어렵다 is a form where the subject marker is elided, which happens often. These threesome structures show up all over the language: 나는 햑생이야 / 내가 학생이야 / 나 학생이야. It takes a good understanding and practice of the language to acquire a sense of the different nuances of these three.

You can use 하기가 어렵다/하기 어렵다. It makes sense. The latter sounds more native. You may compare it to English abbreviations, like 'fyi' 'btw' 'ppl'. when you think of daily chat, no one bother to write 'By the way' thesedays unless it's formal occasion. oh don't forget! 하기를 어렵다 sounds completely foreign. certain things dont get along like 를 in this case.