Soooo...
Stupid explanation to rule out repeated time travel.
It doesn't even try to use the more sensible explanation that you can't train this way because it resets your levels too. This explanation makes no sense either, but at least it's consistent with his first use of it. (Not sure if it had reset again this second time.)
Really bad writing for something that was otherwise a promising title.
Next is introducing "modern weapons" (as an otherworld artifact).
That's actually ok, but I can't imagine it measuring up to planet buster Grain. Unless it's not WW2 Yamato, but space battleship Yamato.
Finally, the association is really stupid. But listening to young Zeff, I could understand. Not taking the advice of their most powerful mages... It feels like the president of the most powerful country of Earth not listening to his medical experts during a medical crisis like a pandemic. It would seem extremely stupid... if we hadn't just witnessed it in the past few months. Sometimes, reality is worse than fiction. If there is a writer for our real-world narrative, he should be fired.
All to say, it's stupid but not completely unbelievable, as long as we assume that the leaders of the association are critically stupid. Or, if they are not, then super-Grain is actually their goal. A type of mad scientist experiment to see if this kind of mental torture can actually produce invincible mages.