Sayaka got killed by Mami during episode ten. Not even a called shot, a sudden attack while she wasn't ever aware and boom, dead. If we have an attack that specifically targets the soul, no matter if it's a big Tiro Final -esque attack or a sudden hit with any weapon within reach, and the only way that could be avoided or deflected in any way was to just say "It missed", it's either going to be stupidly overpowered or so underpowered it never feels like a real threat. I think that's what you call "Balance"