The Frontal Lobe of the brain, as the name suggested, was the foremost part of the brain. It was responsible for conscious cognition as a whole.
It also happened to be the part of the brain that was closest to the eyeball and the eye sockets.
The distance between the opening of the eye socket and the Frontal Lobe of the brain was shorter than Rui's big toe. In a hypothetical scenario, if his toe were to enter a human eye socket at precisely the right angle and location, it would pierce the brain.
That had always been the most lethal option Rui had, from the very start. Yet he lost the first time despite possessing it.
What made this time all that different?
For one, her incredible active and passive defenses were no longer all that invulnerable.
Fae and Nel were using their tremendous power to seal her arms and shackle her neck and head in place.
As for her passive defense, her remarkable constitution wasn't particularly all that relevant here.