Why doesn't he just ask her instead of "going with the flow?" "Hey, that for the sake of the word stuff, can you elaborate?" See? Easy. Author-san, wtf are you doing?
ah yes, mc just saw a a girl in nude and he does typical "i have spells that can alter the laws of physics but have little-to-none for mind-calming spells" and easily overreacted. also lmao it's one of those that gives expositions in a weird manner "such a huge demon.. the only way to beat it is to use the gram" uh sure but why do you need to say it as if you're explaining it? lmao wtf i can easily understand why this is just 6⭐ but imo that's being generous since this is 3⭐ at best
So he jumps 990 years into the future and just happens to enter that cave on the exact day, hour and minute that some girl shows up in that same place. I get the feeling the author isn't going to elaborate on how such an incredible coincidence occurred.
And then there's the whole issue of the MC not asking the girl questions right away about the missing black sun etc. This is some bullshit writing for sure.