Infinite MP + Turn anything into anything. He's basically a great sage at this point. If he needed to use holy magic, he just needs to use sunlight + MP. If he needs dark magic, his shadow + MP. If he needs air magic, it's plentiful. Water? Just spit and compensate with a ton of MP. Fire is a bit trickier but who cares at this point. Lightning? He's a modern man, he knows that we have a nervous system that effectively has some sort of electrical impulse. So he just needs to use that tiny amount of electric impulse to his fingertip nerves and compensate with a ton of MP to create a giant thunder bolt.
As for the city building. Just pour in a ton of MP and literally just reshape the entire landscape into a metropolis with gigantic walls, preset ballistas and other siege weapons and while you're at it, form a giant pristine road leading to where you used to come from because we've yet to establish any range limits.
We're basically full on speedrunning after having glitched out the value on MP and either overflowed or underflowed it and looped it right around back to infinite values. Whether it's the MP itself or the MP regeneration or both.