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
兼ね合い is read as かねあい (ka ne ai) and means equilibrium.
かねあい
ka ne ai
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