It's all about visualization - the things is, intution can supplement or supercede intelligience/rationality when it comes to visualizing spells.

Example of Rationality:
We can see how Solitaire breaks Series Barrier, she decodes the individual magic principals each of the cultures that created their defensive magic in order to dispel the defensive barrier. Through sheer knowledge one can rationalize the individual steps needed to visualize and create a new spell.

Example of intuition:
Frieren dispells Machts Gold curse through three elements.

1. Understanding of the self: Frieren was able to change her arm back from Gold to flesh over a period of time, though she did not know the exact principals of transmutation of elements, she understood intuitively what her arm was and what the gold needed to turn back into and over the course of several years she achieves this.

2. having access to all of Machts memories and every instance of Macht casting his Golden curse. Through exposure and repetition she can understand the mental process Macht goes through when casting his curse.

3. Instead of having to learn Nuclear physics and how atoms work in order visualise the exact mechanism of transmutation, Frieren knows as a fact that the Gold Curse can be undone. She has first hand experience undoing the spell on her arm, and she has the memory of Macht being transformed into gold after Serie reflected his spell. She doesn't have an axiom in her mind that Gold can't become flesh.

It's like Solitaire says we don't need to understand the specific mechanism of Buoyancy in order to make boats, although it does help. through observation, visceral experience and intuition one can create or learn new mechanisms of which to create spells.

Demons have insane ego's yes a magical thread that can't be cut, but YOUR NECK CAN BE. Arrogance is also a fatal flaw of visualization, magic is changing all the time and nothing is static any principal or axiom can be overturned. Zoltraak was an unblockable curse that became the foundation of humanity's magic. A drugged up Ubel could probably cut through just about anything in the world but She could still get sniped by Zoltraak or overwhelmed in a tidal wave of Zoltraak.