I am putting this out because I want suggestions from the TMP community.
The campaign would require an Anglo-Prussian army in 15mm, a lot of 15mm buildings, other 15mm terrain and scenics, and 2 54mm Napoleons. One would need a lot of conversion work.
The first indication that the Anglo-Prussian alliance is in trouble would be a survivor from a wrecked fishing boat with a story about a giant Napoleon. Soon enough, there would be a skirmish between some dragoons with a galloper gun against the 54mm Napoleon. Napoleon would not be seriously harmed by the 15mm forces.
The royalty of Europe would want to know what happened so they send Dr. Kramer and his beautiful lab assistant Siroku to Elba. There they find a mysterious radioactive meteor and surmise it caused Napoleons growth.
Blucher and Wellington realize that ordinary weapons won't harm Napoleon so they mobilize 100 pounder Parrot rifles and other advanced weaponry against him.
Dr. Kramer and Siroku assemble a giant steam powered robot they call Mecha-Napoleon. It chases Napoleon off after a brief scuffle, but wanders off by itself. This convinces Wellington and Blucher that Mecha-Napoleon will turn against them. They decide to kill both giant Napoleons, despite Siroku's tearfull protests. Hussars come riding into Blucher and Wellingtons headquarters with the news that both Napoleons are moving towards Antwerp.
The entire Anglo-Prussian army along with several siege guns, are laying for the two. They fire on Napoleon and the huge parrots knock Napoleon on his butt. A hundred pounds is a very heavy bird, after all. But the weapons have no lasting effect as Napoleon wrestles with Mecha-Napoleon. Buildings are toppled as they throw each other around. Finally the two fight their way into the sea. A submarine volcano broaches the surface and the two monsters disappear.