1. Quite good or impressive. (A colloquial spelling of "hell of a.") That was one helluva a party you threw for Kelly. Everyone said they had such a great time. He's a helluva ballplayer. I wouldn't be surprised if he made it to the pros someday.
2. Very difficult. I had a helluva time trying to change the flat tire. The lug nuts didn't want to budge.
See also: helluva