千番に一番の兼合
せんばんにいちばんのかねあい
something so difficult that one doesn't know if there's even a 1 in 1000 chance of success
千番 is read as せんばん (semban) and means thousand times.
せんばん
semban
せんばんにいちばんのかねあい
something so difficult that one doesn't know if there's even a 1 in 1000 chance of success