Wait I never realized this was a novel adaption, so it makes sense it stopped here. I thought the story was just this much and was even bummed that just this much is what cost future Saika to lose her world!?
From the looks of it though, this is the start of the reason. Still bummed though, I wanted something more epic, and a human mage just taking on the power of an already defeated disaster is certainly not it. Even if the Ouroboros does revive and is more powerful, Saika has already defeated it once and it's not like Saika would've got any weaker to not be able to defeat it again.