Daimon was too tired to fight with the crazy girl right now, that discharge of mana from the door forced him to use Core Synchrony and Sanctuary as well, and even after that he was left with only around 15% of his mana reserves.
That is almost two times the mana reserves that a really talented middle stage Archmage would have, sure it doesn't sound that amazing when there are Stellar ranks around, but when you put into account the fact that Daimon's mana is among the highest quality ones, and that it was used up in a matter of seconds, things change.
If an average true middle stage Archmage would have used all that mana in such short period of time, in a single go, fainting would have been the best result, while damage to its mana circuits would be a much more realistic result, meaning that the challenge to open the door was supposed to be taken by people above the middle stage of the Arch rank.
