Helluva is a written form of the relaxed way people say hell of a. You use it to push a noun: someone or something is very good, very bad, very big, or simply a lot. “That’s a helluva story” can praise it or just mean it is huge and striking — the next words and your tone do the work. It is informal, spoken, a bit American, and a bit cheeky. Not for essays or polite letters. Though actually older writing usually keeps the three words hell of a; the one-word spelling is how the fast mouthful looks on the page. It is mild swearing, so some listeners will mind.