Before he knew Seers were a thing, he thought he was a "wizard with a quirk". I think it's safe to say that he long ago decked himself out in all sorts of wand-like devices to enable him to cast an arsenal of spells.
I don't think what Arthur can do is of any meaningful use in assessing what Magus might be able to do. Arthur is a seer. One established trait of seers is that they have enormous amounts of magic energy that they can put into the spells they cast (via wands), to the point that it had Pandora going on about Tedd having the power to bend worlds - plural - to his will. A seer being able to overwhelm someone's magic resistance doesn't really say anything about how anyone other than a seer might do.
Good point. If Arthur is using that "power to bend worlds" here he would be definitely more powerful than Magus ... also, it might explain why is Terra usually beating Magus. Assuming she's also seer of course.
... unlike just age, which may not prove that much, giving that Magus can easily have better training. Yes, after junior high BATTLE MAGIC school. Or not, of course ... we don't know much about what training Arthur got. It was just speculation that he was jealous of Edward and Noriko being trained by Adrian.
Unlike liberal art schools in our universe, battle magic school is teaching something which can be measured and I'm sure they wouldn't bother with school which is worse than being self-taught. So, Magus's claim of being better than Nanase is believable and he may even be better than older magic users with worse training in EGS universe. Hawk to mice, as Pandora said, comparing Magus to Edward and agent Wolf ...
You really like to accuse me of stuff like this.
However, I refuse to submit to your trick and I repeat that it's obvious he doesn't need to be demigod. Nor the seers are demigods - at least, they were not described that way, although you can say that any son of Zeus (formerly known as Jerry) could claim the title of demigod ...