So his plan relied on the demon lord stating the exact location of his own core? What if he'd said "Impossible! I killed everyone who worked in the construction of the castle! How did you find the core?!" and didn't blurt out the detail itself? Or if he didn't take the bait in the first place? After all, Astaroth hasn't interacted with him enough to be able to prefict his moves so perfectly. The argument "he always stays back so he's never been taunted" is dumb, he had to rise to his position somehow, and he should have plenty of experience being taunted by opponents for his exact strategy. It's the first thing all the muscleheads would taunt him with, he should be used to hearing it from the mouths of the people he trampled underfoot. The first thing that strategy requires is resisting the impulse to head out yourself.