"No matter how you look at it, completely limitless mana is impossible…"
I started with something completely obvious. However, the other mages in my party were listening quite seriously.
First of all, in magical theory, having limitless mana was the equivalent of creating something (mana) from nothing. That would be within the realm of the Creator Goddess's power of Creation, something completely unreasonable.
And looking at it from another angle, if Conservation truly had limitless mana, she could simply toss out limitless forbidden spells against us. Why go to such trouble while fighting us?
If someone truly had limitless mana, there would be endless tactics available to deal with any enemy. For example, creating a limitless number of artificial bone giants. Even if the bone giants would never have enough power to instantly kill a SemiGod, the power of infinite numbers could slowly defeat anyone.