Translation:That job was very hard.
大変 is more formal and.. dramatic, I suppose, as 大変 also means "grave", "dreadful". In a sense it sounds more like the job was difficult to plow through, whereas with 難しい, the job sounds like difficult because it was complex.
It's difficult to explain - 難しい.
It's difficult to renovate the whole garden in just 3 hours - 大変.