Sears sells a high-wheel string trimmer that would do that job well. (1) It's wheeled, so you don't have to carry it; and (2) The string is always at the same height. I use it in an area with a lot of trees and shrubs where my lawn service won't cut the grass, and where I'd have trouble with the lawn tractor.
It uses replaceable string sections rather than a "tap" system, and it uses a large-size heavy duty string. If one of the two strings breaks, it's easy to fit a new one onto the clip.
The two disadvantages ares that you can't turn the head 90 degrees to do edging.and if you're in an area with a lot of vines they sometimes can wrap around the spinning head. -- Regards --