@AnonymousWeeaboo He's already an S-rank, this is a mock battle to demonstrate the power of these S-ranks, which also proves why they deserve that rank. They also both have a magic barrier the priestess cast on them to prevent deathblows, so they shouldn't die until after that barrier has already been broken. They're trying to kill each other because Kelvin is battle crazy to the extreme and wants a life or death fight but, I'm not sure if she's the same as him or if she has her own separate reasoning to try to kill him.
@Echonic But magic like that is pretty common, especially in things like video games and it requires a lot of mp to use. It even drains the priestess that casts it to the point she has to chug magic restoration potions. It is in no way perfect, which is what Mary Sue-ish implies (Mary Sue=Perfect fictional character that can do no wrong).