a single 25Mt bomb would totally destroy everything within 16.9 miles of ground zero assuming a airburst. (that dosen't include the outer area where the overpressure is under 3psi )
so, according to what I have just read, I can make a 901 square mile area uninhabitable with a single 250mt bomb (thats around twenty times less powerfull than the Harbinger than we are looking at)
I would just drop a few dozen harbingers and take my chances with hitting something.
Hey, I don't even need to hit my target. After a couple of explosions like that there's going to so much of the planet left permantly uninhabitable that whoever I missed wouldn't be able to survive for long anyway. If the explosion wouldn't get them the radiation would (not to mention the EMP's going to fry anything electronic so they can't do a runner with a ship)