I use one of those weed eaters on wheels, 6.5 hp, .155 line. Or a brush hog on the larger areas. 39 hp John Deere, five-foot brush hog.
If you tank mix 2,4,D with your Roundup, you get the quick killing power of the 2,4,D combined with the strength of the Roundup.
Roundup sold at farm supply stores is typically 36 percent. At a garden supply or handyman store the best is usually 18%. The "ready-to-use" is 1% and it can still be diluted about 10-1.