For very specific spot killing of problem weeds, I have been successful using boiling water. I have a lovely clump of phlox that had a dandelion growing in the middle of it for years. The phlox was rooted so tightly around the dandelion root that I could never get it out without ripping the phlox apart (which maybe I should have done). So one day I dug down around the dandelion root as carefully as I could, pushing the phlox roots away as best possible, and then carefully poured an entire kettle full of boiling water over and down beside the dandelion root. Result: cooked dandelion root, saved phlox. YMMV.

This doesn't work on Knapweed, which has roots that go across and down to hell.