Question, if I used the word “causality” would you have nitpicked that?
Look man, it's not a nitpick because it's not unimportant. It's not that big of a deal to admit a mistake, you know? But you do you.
Anyway yes, actually, it would still be wrong to say "Guts is the strongest foe against causality". The problem here is that you're ascribing sentience to a neutral force.
To recap: the word "fate" in Berserk is used like in the real world, not to describe some mystical power but more as a poetic way to evoke hardships or happenstance. Causality however
is described as a force. Let me cite myself here:
The concept of causality (因果) is prevalent throughout Berserk. It is often confused with that of fate or destiny (運命), but they are different notions. Causality, or the principle of cause and effect, holds that current circumstances are the result of past actions. Both terms are mentioned in the manga, but only causality is presented as a law of the world; a force to be reckoned with.
The first time we hear of causality is in volume 3, when the Count's despair activates the beherit Guts is carrying. Void refers to the Count as one who has been ordained by the principle of causality. This is a fundamental notion in the story: the fact that these ceremonies and the people involved in them are predestinated. Causality is usually associated with the God Hand and their master, the Idea of Evil. We are repeatedly told and shown that these evil beings make use of that principle to subtly influence the world over long periods of time in such a way that it conforms to their desires.
For example, that's how beherits always happen to be right where they need to be when someone is in utmost despair and ready to sacrifice. It's also how as a human Griffith triumphed at every turn and seemed unstoppable, until he suddenly wasn't. And it's how every piece of the puzzle came neatly into place for the Incarnation ceremony during the Conviction arc, or how Midland was ready for him to play the savior and unleash Fantasia in the process.
What's important for our discussion is that causality doesn't have a mind of its own, it's just a tool for Void and/or the God of the Abyss to achieve their designs. And as I go on to say in that post, it's not their exclusive domain either, it's more complicated than that.
I think you know all of this already, but you're clinging to the old misconception that "Guts is outside fate" or that he's exceptionally able to go against it. If you just let go of it, you'll find things a lot easier to make sense of. And it doesn't change the fact Guts is badass and iron-willed. It's just that he's not impervious to causality (which is pretty clear from the story), and causality isn't the big bad guy, but a means through which the bad guys impose their will upon the world.
This is perfectly summarized by one of Void's famous lines:
"If it's a principle that fate transcends human intellect and makes playthings of humans... It's causality that humans confront fate with evil." What he's talking about is the use of evil power by humans to gain a higher form of agency over their lives instead of being victims of happenstance. It's exemplified by apostles sacrificing the people dearest to them to reverse their misfortune.
On that note, one thing not many readers know is that the concept of causality exists in Buddhism, where bad deeds result in bad outcomes and good deeds in good ones, a notion related to that of karma. I believe this can be useful to understand the nuances of causality in Berserk.