We are visiting Tainan from 30 Aug to 4 Sept, will there be any typhoon or heavy thunderstorms?

According to the weather forecast, the typhoon will influence this are in some degree. But maybe it won't have frequent heavy rainy days and thunderstorms. The climate still will be hot and moist. It's safe for you to tour around in downtown area. However, it's not advised to mountain area, such as Guanziling Area, the area you mentioned.