The worst part is that Tatsuya didn't even get a difficult fight before this massive power-up: it's like "ok you won your last 15 fights against mooks easily with no threat to yourself and without even breaking a sweat, clearly, right now you need to go Super-Saiyan".
Part of the weakness seems to be in the set up. Mahouka is premised where people with magic are the aristocracy, and a common thread in the plot has been that the baddies are non-magically-talented people who want to usurp the "rightful" place of the magicians in society: when Tatsuya was discriminated against for not showing powers, the injustice was NOT that people are discriminated against, his sister pointed out that there's a line, and Tatsuya had been "unfairly" tagged as being on the wrong side of the line. In other words: what was injust wasn't the elitist system, it was that Tatsuya should be counted as an elite, not a pleb.
It's like a story about a prince in rags, and how unfair it is to treat him like a peasant, if only they knew "who he was" he would "rightfully" be treated like a prince, without actually saying that it's inherently wrong to mistreat peasants: they're not "rightfully" princes now are they?
The times he's fought anyone really magical was just sporting events, and that rival wasn't really bad or trying to hurt him. So battles between the magical elite are shown as gentlemanly contests rather than outright combat.
Since the enemies are by definition talentless plebs, the series has a hard time giving any of them any personality or unique traits. Mostly it doesn't bother trying and has mooks in grey overalls for 99% of fights.
But this magic vs non-magic divide creates a big problem for the series, because basically all the enemies are non-magical wimps who use guns to try and tangle with OP wizards. Very few of the enemies have any magical powers.