Yeah, Cyn would absolutely wipe the floor with the Radiance. I also heavily recommend Hollow Knight, its insanely fun and pretty long if you go for 112 percent completion.
While yes all creatures in Hollow Knight are bugs, they are much bigger than their real-life counterparts. And your claim that the knight wields a "nail" is correct, the nails within Hollow Knight are not actual nails, they are instead swords that are referred to as nails. However, I do agree with you that the Radiance would get destroyed by Cyn.
To clear this up, the Knight is canonically 1 foot 5 inches tall, and by measuring the sprites of the knight to Absolute Radiance, it turns out that Absolute Radiance is about 10 feet and 6 inches tall.
