I would say a scythe or a bow knife. Try looking at Lehmans.com, they sell alot of hard to find items like hand farming tools.

I've spent a a couple whole days this summer, doing just that thing. I used a grass whip. It was a lot of work that would have been prevented were I not so afraid of the brush cutter attachment for the weed whacker, but it works fine and gives you some practice for your golf swing, if you play!
deb

I use a scythe with a brush blade. I bought my snath and two blades from Scythe Supply. They'll use your measurements to make a snath that fits you.

Here is a link that might be useful: Scythe Supply

Hey, thanks for the Scythe Supply link, Robin! It's hard to get new scythes from our local hardware.

We have running bamboo, ten to twelve feet tall clumping guinea grass and reznor grasses which get all out of control. A machete doesn't work well on these things at all. Machetes do well with small trees, but not grassy things. Might have to do with lack of weight and inertia. Sometimes I use the scythe - get it real sharp and revel in the Halloween "Grim Reaper" look. Othertimes I have a chain saw blade on the weed whacker and that works well, too.

The other option is goats. Fence it in and see if you can borrow a herd of goats.