Femradiel insisted on attacking the dragons and eventually discovered at least one of Ymir's weaknesses when it comes to summoning magic. He edited his monsters skills that had a limit. For example, the spectral dragons began to die when they suffered enough damage… Somehow, Ymir made them to be like that to make everyone doubt their power. The monsters obviously didn't become invulnerable to time magic.
"They didn't become immortal! They just changed to become more sturdy than before!" Femradiel shouted.
As to why the monsters couldn't go to the Void World anymore… it was hard to be sure, but Femradiel assumed that Ymir copied Hypnos' traits and then passed them to all the other copies. It seemed like one heck of a skill, but she already knew the basics of the Absolute Summoner class to know that such a thing was possible for Ymir.
"Your ability to analyze a situation is quite something," Ymir said. "But how about this?"