Interesting, my absolute favorite marvel character vs someone I know nothing about.
I think I'm gonna lean in Guts' favor in this match. The Black Swordsman without Berserker Armor has shown more impressive reaction time than Cap as he was able to fend off the supersonic speed attacks of the Apostle Rosine and I believe he's shown better feats in the strength and durability department as well with his casual use of the Dragonslayer in combat and the freakish amount of punishment he's taken from Apostles like Wyald and the Snake Baron. While the Supersoldier Formula does push Captain America's physiology beyond peak human limits, I don't believe he is quite near Guts' level of physical stats. I will agree though that Cap's Shield and his more extensive battle experience will give Guts a bit of difficulty in this match. However, Guts is well known to adjust his tactics on the fly in the heat of battle and is capable of delivering attacks that surprise his foes. While Cap's shield would help him fend against all of Guts' attacks, I don't picture the body armor worn under his costume holding up to too many blows from the Dragonslayer and he won't be able to use the shield to guard against all areas of his body. I can picture Cap dragging the battle out via his shield and experience, but I don't think he can take out Guts and it would be a matter of time before he faces defeat.