Caroline IS a super human, but barely reaching it while Faust is on the upper echerons of super human power.
Is a nice remembrance that Faust might be strong but is in no way invincible and other people can be a real menace to him. Normal soldiers are not an issue, but trained knights CAN kill super humans in the setting if they overhelm their foe
Faust is not on the level of Guts, on whom supernatural beings cant believe hes just a human because trascends his humanity that much. Serpico is the troublesome human foe for Guts and even him is not a real threat. So yeah Faust is kinda an expy of Guts but they arent on the same level compared to each one settings
By the way Caroline got gutted with that blow, the slash went upwards but the damage was on her abdomen and was less deep on the chest area. Also Faust weapon was originally an oversized ceremony blade, yes it was enchanted but was too big and cumbersome to be used in real combat. Faust due to his size and muscle mass can wield it one handed if he wants and became his weapon of choice
I mean, it was too big and heavy for regular people. The first Lord Polidoro
won it in war with Willendorf. Unless something happened later that I hadn't read that explained the exact source to just be a ceremonial blade, it was made for use by a super human. His weapon in this manga is basically just a thicker Zweihander.
I'll preface this by stating I'm not anything but an amateur at Longsword fighting:
I think the big problem in direction is how he went from, using Meyer stances, what looks like a Schlussel (Key) guard to an opposite Ochs (Ox) or Einhorn (Unicorn) guard. That change wouldn't really allow the cut to diagonally cross the enemy's body. That should have been a thrust. If the starting 'blow' had been from something like a right side Nebenhaut (Tail) guard, you see the position when he starts to gouge the ground, to the left hand Ochs he ends the fight in, the cut would make sense.
It just doesn't flow properly at all in the sequence. The fight needed an extra couple of panels for the exchange to make sense. If he transferred from the Schlussel to the parry as he did, he could have continued and moved not to the left side Ochs but a left side Zornhut (Wrath) or Tag (Day/Roof) and wrenched her polearm out of position. Then we could have seen a panel or two of him actually cleaving from a Zornhut through to an Eisenport (Iron Gate) or Nebenhut.
You can see a decent transition from an Ochs/Schlussel into a quick Tag that carries into the Eisenport that would make a similar cut. Should be the first example after the Eisenport if the link somehow fails to take you to it.
Tl;dr; You aren't wrong to be confused, because it doesn't make sense! But at least the Mangaka is trying with proper guard positions.