"It is hard to write essays."
According to my dictionary "ぶんしょう (文章)" means also essay. So, what's wrong with: "ぶんしょうを書くのはたいへんです。"
作文 is made out of 文章. 文章 meaning is more similar to "sentence" than "essay"
If to write is 書く is 書くの writing? So why does this sentence want 書くのは instead of 書く. If it wants to say write why does it want writing in the answer?
I as understand it, this form effectively turns the verb into a noun. Right here, 書くの is the subject of the sentence, so は is the particle here.
(noun) は (adjective) です
Maybe it's easier to understand if you rewrite the english sentence as"Writing essays is hard". たいへん is an adjective so the first part of the sentence has to be in the form of a noun (書くの) instead of a verb (書く).
I think (but I am not sure) that putting the の after that phrase makes it "-ing". In this case, putting の is sort of like saying that "write an essay"-ing is hard.