....you can equip the counter materia...and let him wack you one time.
or you can just doubleclick on ending2 and then ending3 on your last disk, in the movie directory. You won't miss anything from the game in doing so. Just put your mediaplayer in Full Screen Mode.
Or you can do a trial and error type of thing until you squash the bug....but I don't think anyone has found a solution to the omnislash crash, yet (not everyone has it).
You can try putting your directsound into basic acceleration mode. Type Dxdiag, in the run thing of windows, then go to the sound tab. You don't need to reboot for it to take effect.
You can try it in Softwaremode half screen size. (but what's the point?).