Disintegrate the Hostile City PeAnAqAuCoHeIgImTe
Level 120 Ritual (Base 30, +4 Arcane Connection, +4 Boundary, +3 Size (so 30,000 feet diameter), +7 for extra forms)
Destroys all animals, water, air, people, plants and solid objects within the targeted location. The corpus and animal forms give the base level; it might need to be increased if the SG considers disintegration to be more than just cosmetic versus 'killed'; the other forms use the word 'destroy'.
If you also want ghosts, faeries, demons, and magical beasts destroyed then the spell becomes more complicated: it depends on how high a might you want to destroy. Eg simply adding 2 more magnitudes to affect Mentem and Vim gets you to L130 and will destroy up to and including Might scores of 40 (base level is 30, plus 10 = 40 might).
One I like better is:
Removal of the Inconvenient Urb ReAnCoHeTe
Level 105 Ritual (Base 35, +4 Arcane Connection, +4 Boundary, +3 Size, +1 extra form)
Instantly teleports the target city to any location that you have another arcane connection for. This version will leave non-human, non-animal creatures behind.
Penetration is less important for this one, since you've removed all mundane things - destroy what's left with the spell(s) of your choice, then teleport the city back again if you want!