I don't really get what you mean by 'other magic'. Like, what magic would Anders be using to begin with? Because he's using SOME magic apparently.
Whatever the outcome, my other points still remain. If he's supernaturally smart, he can't really just magically infuse something with fire or something.
But the point is, he's using super smartness and wizard skills to figure out
how to do stuff like infuse things with fire.
How does this sound?
Using Anders' passive to allow him to reverse-engineer magic (and apply it to items?):
Level 20 Analysis:Anders reverse-engineers one spell cast up to two turns ago. He can either immediately re-cast it, at 1 level less than its original power, or, after the end of the battle, add it to his Known Spells List.
The Known Spells List: Anders' phenomenal intellect allows him to, while out of combat, look through all the magic he has studied and select some of it to prepare for the next battle. He can prepare up to [INT](20) levels' worth of spells with a downtime of 30 minutes in which no strenuous intellectual activity occurs (i.e. talking is okay, cryptography not so much). He may alternatively swap out [level] worth of spells in [2*level] minutes to a maximum of five levels, even while in combat.
The casting stat for all Known Spells List spells is INT, regardless of what it was previously.
So stripped of flavor:
Anders can either re-cast a spell from up to two turns ago at a -1 level penalty, or add it to a list.
He can have up to 20 levels of spells from that list active, and casts using INT.
It takes him 30 minutes to prepare the list out of combat.
In combat, he can switch out one level's worth of spell per 2 minutes for up to five levels.
He casts spells from that list.
Also, so that he doesn't just make a level-20 spell and cast it twice to omnimurderdoom everything, I have an idea that if the spell is higher level than his WIL, he needs an extra turn per some fraction of levels.