Terrain in North Korea is mountaineous and this gives a million man army of North Korea advantage in defenseAs said if you remove the rukes outside of the picture South Korea should be able to run thru North Korea quite easily due to just being more heavily armed then their neighbours
Their soldiers are highly motivated and armed with ATGMs, anti-material rifles, MANPADs, and hold defensive position in the mountains filled with thousands of hardened bunkers and underground tunnels
Also they have a very large number of artillery pieces and thus massive firepower.
They employ decoys and camouflage to distract enemy army and are entrenched in the mountains that provide advantage for defense
Their million man army hold a very strong defensive position and cracking them will not be easy.
Their air defense systems are obsolete but they are very numerious and suppressing them without US air force support will be very difficult for the South
South Korea military can't defeat North by its own--support from USA is required
Just an example of a similar war is Israel-Hezbollah war of 2006-----as you know Israel failed during that war despite its massive firepower