There's no taking on NK militarily without a major disaster of some kind being perpetrated on Seoul. The NK has hundreds of artillery batteries hidden away in the mountains trained on Seoul and they do have the capability of turning it into a "lake of fire". There's not alot that can be done about it besides carpet bombing every inch of those mountainsides. That's not to mention the size of their army and manpower, sapping, tunnels, infiltrations, etc.
An all-out military invasion of Nk is out of the question. Unless they get so worn down economically that their army gives up immediately after commencement of hostilities.
The only way to approach them is diplomatically, which is finally coming about. My honest opinion is that they will eventually lose more support financially from a more liberalized China and will eventually see the need for reform.