dejectedgeek said:Guts was just a really strong person to begin with, before the Eclipse stuff, since he trained at a higher level than whatever he was supposed to at his age. Living in the Interstice has just made him more attuned to that world and so he can see/touch all those things...that's what the manga said anyway.
The Dragonslayer on the other hand has gotten very powerful from slicing through the hundreds upon thousands of monsters and the apostles. Guts himself explained it during the Lost Children arc, IIRC, something about the sword stopping their spirit or something.
It has been more or less clarified as a solid theory that Guts sword has definitely been spiritually amplified as said, but that doesn't rule out the possibility that he hasn't lost some physical limitations from being there for so long.
Even so when Schierke mentions "Is it because he's been in the 'interstice' so long where the spirit can influence it's physical surroundings, or...? The shadow around that sword.... " when first watching Guts kill trolls so easily, it didn't sound like a "It could be this as well" sort of joint theory, rather it sounded more like a "or.... it could just be the sword alone" speculation.
If only one of the speculations was accurate, it would definitely be the second mentioned, and though it's believable his strength increased so much more because of the branding, it is clear that even before he was the Black Swordsman he was ever growing more powerful at a rapid rate as every year passed.
We must also consider all the beings that live in the interstice and how they are more powerful than beings in the physical world alone; Apostles, Flora, Skully, Trolls, Qliphoth, the dead that attack Guts which aren't overly powerful but take into consideration they are powerless in the physical world alone. Guts also lives in the same realm as all these beings so....
In short, I dunno.