In other words "Just outplay them". C9 are not some one-trick pony with a single, strong strategy that falls apart when other teams figure it out leaving them with nothing; they are not limited by their champion pool either so you can't ban them out. There is one way to beat them: play better. No shortcuts such as countering the one and only strategy or taking their champions away, opponents have to simply play better, from the champion select to the last hit on the nexus... and apparently entire NA LCS is a league below them, at the very least, and thus incapable of doing so.
It's funny to see nearly everyone with W:L ratio below 50%, all thanks to C9's 91%. ^.^