For whatever monster you completed adventure mode with. After taking on (presumably) every other monster in the world, you fight the alien leader, and beat the daylights outta him too!
Cities are almost entirely destructible. Have fun.
A good fight can be almost as fun to watch as it is to play, with buildings crumbling, monsters being flung across the cityscape and the devastation causing everything from earthquakes to volcanic eruptions.
Your monster of choice apparently thinks nothing of surviving a close-by nuclear explosion and then jumping into the smouldering crater to kill whatever it may find inside during the campaign.