It was not a matter of possible or impossible from the start.
Rex is already determined to utterly destroy the Rastrikan Demons' forces and image, he wouldn't fail no matter how hard it will be to reach that goal, he's confident that there's nothing that could stop him from getting what he wants.
A confidence that has been tested numerous times.
Despite the doubts that he was given by everyone else, he has never doubted himself.
From the moment he planned on showing the world that even the Rastrikan Demons could still be conquered, his mind already raced and concocted numerous plans, coming to a conclusion of one solid plan that would not fail.
Out of everyone, he was the one who believed in the Demon Lords' willpower.
If he wanted to force them to kneel before him, then he would need to accept this fact first.
Just as predicted, Demon Lord Aructh and Mazel rather die than kneel.