If Astralious truly merged with the Ice Phoenix Flames, then Elias could easily suppress the fragile consciousness of the flames and make them his own.
Even if Astralious were freed from the prison of his mind, he could still be controlled through the Creation Control Skill, as long as he was merged with something like the flames.
Elias didn't have to worry about losing a precious material like the flames.
The flames trembled suddenly, as if they could understand what Elias was talking about. Based purely on instinct, the flames grew even more scared.
They didn't want to lose their consciousness, or the semblance of freedom that they still possessed. To protect themselves, they moved straight to Elias's hands and accepted him as their master, even though they were still scared of him.