This may not be necessary and may not be the response. Right up there in death toll with the Hiroshima (140,000) and Nagasaki (70,000) bombs was the two day firebombing of Tokyo (100,000+ dead)
Modern conventional weapons have advanced with fuel-air explosives and other Thermobaric weapons There is Napalm B, also called super napalm. It burns for 20-40 times longer than regular Napalm
cluster bombs and large conventional bombs are also available in the conventional arsenal.
If the medical, utility and transportation infrastructure of an enemy is destroyed using conventional weapons then disease and starvation would rack up a big toll.
Clearly this is not something that anyone would want. Therefore, terrorists would be miscalculating if they thought positive objectives would result from the nuking of a US or western city.
The thing to understand is that whatever death toll is desired, overwhelming conventional can get you there if you have air superiority. Is there any question as to which country will have air superiority?
Against the Soviets the response to nuclear attack was counter nuclear attack. The reason is that they had far more weapons available so you needed a fast destructive response to try to limit the damage being inflicted upon you. If the new terorist enemy has mostly shot its load in the first wave, then you can take your conventional time of days, weeks and months extracting your price in response.