I've cleared acres of multiflora rose and rusian olive, some so thick a rabbit couldn't get through it. There are basically three methods; animal (cattle, goats), mechanical, and chemical. I've used the latter two methods.
Mechanical:
Chainsaw, chain and drag (big stuff)
Tractor w/ box scrapper used like a dozer - most effective in spring with ground is soft and roots are loose/ pile it up and burn or leave for wildlife.
Tractor w/ hog - smaller stuff and maintanence
Results - successful at a price...I've torn the $4it out of my tractor. Wear Carhartts and prepare to give blood.
Next time I'll rent a D3 size dozer.
Chemical:
2,4 D mixed w/ diesel in early spring (goes through the bark)
Round-up in April and May after leaves come out (growth).
I've a 14 gal sprayer on the back of a quad. This works well on fencelines.
Your mileage may vary.
Cheers, edski