Finding Rex's location is not that hard for the Witch of Chaos to do.
A small portion of her cursed energy was embedded inside Rex's body to act as a tracker, in case she needed to locate him as soon as possible. It was within Rex's consent, as in the motion of his plan, the two of them are on the same side.
So there's no problem in letting her leave a tracker on him.
Moreover, Rex had already expected her to come, conveying exactly this message.
It was time for the final showdown to begin.
A flow of adrenaline quickly sabotaged his bloodstream and made his body tense.
"Gistella did it huh..." Rex mused with both of his fists clenched, a small sense of relief invading his heart. "I was reluctant to use her, but I'm glad that she managed to pull it off. Witch, when you get there, tell her to take a low profile and prioritize her safety"
"Shouldn't you be able to do that yourself?" the Witch raised an eyebrow in confusion.