He's not faking it

"Ah, for a moment, I was more than certain that you didn't have one." Alice said and then got serious, glaring at him with her cold and deadly eyes that looked like they could kill him if he made even one wrong move. "Now, let her go."

The sand and dust gradually died down and the kids were able to see the dead clones on the floor. And then they saw Alice, whose hand was deep inside Big Stach, which was several times bigger than her.

She was able to find the right location of his heart without any problems with her third eye. And though Big Stach was bigger than her, she just needed to stretch her hand and she would reach his heart.

His body might've gotten bigger but that only made his heart smaller in comparison.

And this happened to be his weak point. Even if he took demon blood, he wouldn't be able to survive once his heart got crushed.