Nobody seems to have defined “Megacity”.
Of the current Megacities noted in Wikipedia, the smallest seems to cover over 400 square miles and the largest over 5000 square miles. That’s a lot of fire bombing or artillery. I don’t know how much nuking it would be.
When the electricity, food and water stop going into a mega-city, the problem will self-correct with no rubble or radiation left behind.
A clever enemy can even shift the blame to a 3rd party. Hard to do with ICBMs or B-52s or 155 howitzers.
Which is why it is much more likely to happen. There is no MAD to a false-flagged cyber attack that manages the top three effects, that is, cutting off electricity, food and water.
A modern mega-city is a tech-construct as vulnerable as a moon-base under a glass dome that runs everything on a nuke plant. Break the dome or shut the plant off and the moon-base or the mega-city will die.