Any strike on NK has to decapitate it instantly, or the conventional artillery along the border will pound Seoul into rubble in the meantime. Ballpark figures are 10,000 pieces, some dug in, some mobile, each capable of firing 10kg high explosive and keeping it coming until their ammunition runs out. It's a hard problem for any military planner.
If the SK capital was on their south coast and it was only farmland within artillery range of the border than NK would be an easy problem to solve (obviously if that were the case NK would have developed other weapons to compensate). Notwithstanding China of course.
You would also need to somehow move your DMZ-overrunning force into position without that manoeuvre itself triggering the artillery bombardment.