I haven't read the WN, but Hyce uses a Minigun at least once in the story, which shoots .308/7.52x51. Before we even consider the effect of
magic bullets, the armor piercing variant of this ammo, M80A1, can easily penetrate almost an inch of steel. So, I don't think a few centimeters matter here.
And also, this is a fight between the
main character and a potential harem member in a power fantasy manga. Plot armor will win every time. You'd do better than to be so condescending when you can easily be made a fool of yourself.
A cool video of 7.62x51/.308 armor penetration if anyone is interested!