Definitely one of those situations where MC being overpowered is forced to be a dumbass and react like a dumbass in order for the story to actually be a story, because let's be honest here, he could've resolved everything before it started if he noticed them already. For !@#$ sake, they said they wanted to kill people and he's still going 'well, let's see what happens'. Like really, if you're gonna get a MC w/o in-universe common sense at least don't make him OP in all possible respect.
couple of minor points...
Yuuji has got his slimes in place.. When it comes to the evil cultists, each and every one of them has a nice little red dot tracking their foreheads as soon as those slimes arrived...
He wouldn't even have to use HoD, given that a "normal" fireball by the proper slimes is
already overkill for humans + equipment. And most of the local scenery...
Similar for the monster floods... He's got the Bird tracking them, and either has slimes in place, or soon will have them along the routes.
If things get too dangerous he can finish them off or delay them with ease.
This would, of course, completely ruin his "undercover profile" , for what it's still worth.
Even when he projects his spells through his slimes, he is
still building his constructs in plain sight for any mage to see..
And what the faculty + students
would see if he would do that in public would be constructs that are, to them, insane, legendary, impossible, pick your poison.
Probably all of it would be described.... In that Library..
Oh ... and the Main Office of those evil scientists is far too well-informed about the Academy, its roster, and any evacuation/defense plans for the cities...
Many-much sus being sus about the whole situation.
By simply waiting until he
really has to act Yuuji gains a lot of info about the Baddies while maintaining his
ahem "Low-Profile Persona" while still keeping people as "safe" as possible.