Seeing as how everyone seems incredibly reluctant to give me a proper description of how this power works, I've looked it up myself. It only activates when used against an opponent's most powerful [i]attack[/i]. This only works if it is their most powerful attack, and is absolutely and completely useless against well-rounded mages such as most of the masters in the series, or, of course, Stein. Furthermore, the blade does not cut it's opponent in half, it rather causes a small hole more-less the size of a pebble, the minimal amount to gauge out the heart. 
There, this has been solved. Now, based on the fact that I want this fight to be over, I will compromise and agree to both characters living, despite the fact that Fragrach should not have been used in this manner, and has been abused in these fights. In the future, I shall react appropriately, and be researching every item Gilgamesh pulls out. Now I need to go calm down.