How should I kill viscious waist high weeds?

If the idea of the flame gun does not appeal. Why not use the weed killer on the brambles & the really tuff stuff. Like where the roots are in the concrete..
If you are going to use weed killer anywhere, whether SBK or Glyphosate types, you need to do it BEFORE you cut it back as they are systemic so need to be sprayed on the leaves first. Then it works it's way down through the plant to the roots & will then kill it. So I would weed killer what had to be done first, then I would leave those plants till last to cut down. I think I left mine for a couple of weeks in the end for the weedkiller to have a good effect then cut them down... Hope that is some help too..