Like, there are two main characters. One has the ability to revert all phenomenas up to 24 hours ago(Da Capo). The other one has the ability to choose the future he desires, rendering any kind of defense impossible against his attack because he just chooses the one where the defense fails(Nothung). Now, the big bad was immortal since 12 years or so, so the two protagonists combine their power to this:
Using Nothungs power the selected future upon the big bad being hit was 'big bad being hit 24 hours prior to actually being hit' this "future" repeated itself over and over again and reverted the big bad to a state where he was not immortal, which ultimately allowed the two to defeat him :^).
That is the video with a explanation better than mine, maybe.