In the first chapter: Shion gatecrashing a wedding on a freaking dragon!
In order to use magic wizards and witches have to accomplish rituals, which could involve fasting, killing monsters or procuring rare materials. Here's a list of what Shion did to get his magic:
Killing one thousand large insect monsters with only a broom. It's even cooler when it's shown that at first Shion failed the Ritual so many times the guy who arranged the Ritual advised him to stop. Of course, eventually he accomplished it thru sheer determination.
And later on he takes on thrice that number. Oh, each one of those insects were also thrice as powerful. Just so he can get his hand on more powerful magic.
Hunting the horns of a fire dragon.
Stealing a bunch of rituals that were state secrets.
Recently, inventing his own damn Ritual so he can have a unique type of spell.
And who could forget the fact that Shion lives in a dragon?
In chapter four, it's revealed that Shion's broom magic is the highest rank of the "Puppet Branch", and Shion promptly demonstrates its power by demolishing an entire island-sized castle.
In chapter 15 Shion uses his broom threads to stealthily tie up the arc villain to keep him from moving. This also allows Rogue to bypass the villain's Attack Reflector magic by letting his fire travel down Shion's broom threads! Kaboom.
CINDERELLA FEVER! It lets Luu turn into an older version of herself, who can punch with enough force to send Balthazar crashing out of the building they were in and skipping across the ocean like a stone.
Chapter 22: Emma as a determinator.
Emma: Neither Shion nor anyone else has given up on me... There's no way I'll give up on myself!
And Emma demonstrates the power of her soul via the Resonance Of Souls, which allows her to revive Shion with enough strength to take down Gaspar.
The link between our souls shall erase the night of perpetual darkness and bring about shining stars.
Chapter 25: Broom power suit. That is all.
Shion's status as an Archmage. Especially when you find out his backstory in Chapter 28.
Shion turning down Faust's "offer".
Shion's fight against Juubee, where he takes on a miniature sun with brooms. LOTS OF BROOMS.
Chapter 41: Joshua turning HIMSELF into a pig so he can act as decoy while his wife takes down wolf-mode Alice with a magic-empowered spoon.