Shingi could detect two sets of auras around his opponent. He had seen both of them as the auras of the Manticore and the female mage, but this time they were merged.
He had noticed that, with his ability to detect his fellow companions' HP, SP, and MP, he could perform a similar action on monsters, too. But he didn't detect the same set of auras on them; instead, it was just a general aura similar to the aura he always detected from Blessed Ones.
If he hadn't detected them when they were separate, he possibly couldn't tell that they were two auras instead of one. But since he was aware of that fact, he noticed that after the last spell attack, the aura that belonged to the Manticore had lost a piece of it, as it vaporized to nothing.
This was why he was confident that she used the life force of the Manticore instead of hers, but he wasn't sure how she reached her current form.