From Touhou Wiki: "Unwilling to let this happen, Suwako resisted and fought with the finest steel weapons of the time. However, Kanako held out a thin vine, and Suwako's large steel arsenal instantly rusted away. Suwako realized the difference in their divine powers and her defeat; she graciously forfeited and relinquished her kingdom.
Previous to that defeat, however, wasn't it true that Suwako fought and repelled all other would-be-conquerors? Kanako was just the first to have a power that neutralized Suwako's strength. Rock-paper-scissors aside, Suwako appears to be her equal in power -- perhaps her superior, since Suwako is the Ex-Boss of their first appearance.