There was no time to waste. He needed to do the most damage he could, as fast as he could, and constantly, if he wished to bring down this beast.
Astaroth immediately cast Ignite on the bear, causing its fur to light on fire, as it started taking damage continuously. Of course, Astaroth knew this damage alone would never be enough on its own.
That was why he had already thought of something else.
Luna's meld was coming undone already, and he didn't want to meld with White just yet. He thought it was best to have all his summons out to help fight the smaller corrupted monsters.
So he tried figuring out a way while he fought earlier to keep them summoned. And an idea occurred to him.
Astaroth recalled the process of summoning them, as he did so with Luna. He would inject mana into their souls, and that would conjure up a temporary body for them, allowing them to manifest.