If FAT BOY is really stupid, you are quite right.
As an armchair quarterback, I would nuke all along the Northn part of the DMZ with 455 megaton bombs to hopefully take out a bunch of FAT BOY's 20,000 artillery pieces.
And keep nuking up the peninsula if FAT BOY has more artillery fire at Seoul.
There are 20 million people in Seoul that could be seriously harmed by those 20,000 artillery pieces.
We can dial our NUKES to be very clean, as I understand it.
And as soon as those NUKES impact, the artillery would not be able to fire through bomb blasts, provided a wall of NUKES are set off.
Otherwise there are estimates of 1 million South Koreans dead from an artillery attack.
The 20,000 artillary tubes has been overstated,
Read Here:
http://www.redstate.com/streiff/2017/08/09/north-korea-level-seoul/