At the same time though who says it has to be handwaving. throw in a spell circle the earth shaking from the amount of power and mana sparks flying and throw in some awesome ancient sounding words and youd get a pretty good scene.Scarcity of magic is one of those things that can be in issue in some stories, or not even mentioned in others. I find systems that address the scarcity of magic are more realistic--you can't pull a giant Deus ex Machina via magic with some handwaving about "Well, it's magic! It's limitless in its power!" Difficulty and scarcity make magic more believable and interesting to read about for me.
but there are many systems of magic.
Hiro Mashimura is the author of fairytail a manga i read and still read and in one of the volumes the chyaracters need to power up fast so it explains how magic works and that it comes from the body and that as you train/mature you get an extra source of magic in their body hence escalating their power.
theres also ways of gaining power like in said book Dragon slayers can eat their element and gain more power.
okay from what i said ill describe a scene.
A spell circle 30 feet in diameter spreads open as a mage speaks forth his spell from his memory he speaks ancient words known only to him and any other versed in the language. as he speaks mana sparks fly and the earth shakes and he focuses his power into the spell. in front of him an arch Demon stands yet does not act for he was binded by the spell and further binded by the chains that were summoned and as the mage spoke more and more of his aincient words the earth buckled underneath the demon and grew upwards to cover it. the mage spoke he last of his spell and a huge world tree grew from where the demon was trapping it for all eternity.