My first battle with Zeromus went a little like this:
Zeromus uses Big Bang
Cecil Attacks
Zeromus uses Big Bang
--End--
Just in case you haven't figured it out yet, I lost.
Took four Big Bangs to take me down on the PS version. Took me a while to figure out that he was probably countering with Big Bang, as all of them came before Rosa could heal me.