It depends on what your definition of "win" is.
Do you need to leave the town in smoking ruins with it population dead? Alterantively is slowly destroying the area and dispersing the population through famine, war, drought or disease acceptable?
If the latter then any spellcaster can do it once they start getting access to control weather. Druids almost certainly have some dodgy plant destroying spells to kill crops or plant growth to ldrive out crops with the native flora. Control Winds on its own (at character level 9) can destroy huge swathes of farmland. Some of these tactics may be available at even earlier levels if you can access scrolls.
Diplomacy based characters can destroy whole regions over time by inciting riots, civil unrest of war between neighbouring regions. This can happen from as early as level 1 depending on the level of permitted cheese.
EDIT: if the challenge is to actually destroy the town and kill everyone there then you must have a spellcaster. Fighter types dont have access to the resources necessary to commit wide scale destruction of property without a degree of investment in magic items. Then its just a case of determing the EL of the major NPC's who will remain in the area. You might want to add a bit of a modifier for the general population but the reality is after a certain level they become mostly irrelevant. The EL would also be adjusted to account for the fact that not all of the NPC's will be able to react at the same time. Given the ease of scry and die tactics I would expect many of them to be quickly picked off one at a time leaving the town or region defenceless.
Do you need to leave the town in smoking ruins with it population dead? Alterantively is slowly destroying the area and dispersing the population through famine, war, drought or disease acceptable?
If the latter then any spellcaster can do it once they start getting access to control weather. Druids almost certainly have some dodgy plant destroying spells to kill crops or plant growth to ldrive out crops with the native flora. Control Winds on its own (at character level 9) can destroy huge swathes of farmland. Some of these tactics may be available at even earlier levels if you can access scrolls.
Diplomacy based characters can destroy whole regions over time by inciting riots, civil unrest of war between neighbouring regions. This can happen from as early as level 1 depending on the level of permitted cheese.
EDIT: if the challenge is to actually destroy the town and kill everyone there then you must have a spellcaster. Fighter types dont have access to the resources necessary to commit wide scale destruction of property without a degree of investment in magic items. Then its just a case of determing the EL of the major NPC's who will remain in the area. You might want to add a bit of a modifier for the general population but the reality is after a certain level they become mostly irrelevant. The EL would also be adjusted to account for the fact that not all of the NPC's will be able to react at the same time. Given the ease of scry and die tactics I would expect many of them to be quickly picked off one at a time leaving the town or region defenceless.
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