Yuji Sakai upon seeing his friend Hirai about to vanish after just one day manages to get her to exist a bit longer by forcing the whole class (but especially Ike) to acknowledge her existence. She still fades away, but it convinces Shana to use her torch to take over her identity.
On that note, "Hirai" (as possessed by Shana) becomes a holy terror for all the teachers of the school, telling the first teacher to study the subject properly. Perhaps this is why in the second season, everyone seems to spend most of their time goofing off rather than studying.
The typical MacGuffin-carryingOrdinary High-School StudentNon-Action Guy, who has been something of a Distressed Dude for most of the show and slowly developing his powers, finally gets some respect from the fans when he kills off Zarovee's five selves one by one, actually breaking one's neck with his bare hands. Granted, Yuji's power was explicitly stated to dwarf Zarovee's (who had been relying on a hostage situation), but nonetheless, he crushed a guy's throat in with his bare hands.
Then in Season 3, he makes an entire army retreat in tears with a speech. Think about it. He routes an entire army, by politely informing the Flame Hazes that they are fired.
Really, a big part of the series is about Yuji taking a level in badass. By the end of the first story arc (the Friagne arc), he had gone from Ordinary High-School Student who had traumatically learned the truth about himself and the world to somebody whose insights and advice proved instrumental in defeating Friagne. In the Aizen sibling story arc, he worked with Margery to identify and destroy the siblings' power sources. In the Dantalion arc, he first used Blutsauger in battle. By the second season, the pace of his training, at the hands of both Shana and Wilhelmina, has accelerated, and he becomes powerful enough to handle minor threats like Zarovee and even land a hit on Shana while sparring (and it is noted that his potential is equal to that of a Crimson Lord). By the end of this season, he has begun to fight alongside Shana in actual combat. By the beginning of the third season, he has merged with the Snake of the Festival to become a Physical God and leader of Bal Masqué, capable of matching Shana in combat. He cripples the Flame Haze Army, and out gambits Shana and her little band of Flame Haze during the final battle for the fate of Xanadu. He gets his own unique spell, Grammatica, which is capable of doing pretty much anything, which he retains even after he and the Snake of the Festival split.
Wilhelmina is hinted at being fairly strong, but you don't realize her true power until episodes 20 and 21 of season 2, where she battles the implacable assassin Sabrac, dealing blow after awesome blow even after being struck with gradually opening wounds. The battle includes faking Sabrac out with a fake Yuji, setting her bandages to explode like magical remote mines multiple times, and crushing Sabrac between two buildings and then nuking them. The fact that Sabrac, The Destructive Blade, take all of this abuse without even flinching (and with amusing self-criticism) makes it a Crowning Moment of Awesome for him as well.
The entire cast gets one in season 2 episode 21 when they finally take down Sabrac, complete with an utterly awesome Theme Music Power-Up.
Episode 13 in the first season give us two Crowning Moments Of Awesome, Shana's My Name Is Inigo Montoya and Margery Daw's Big Damn Heroes scene, both of them the turning point in their respective battles.
Khamsin and Rebecca's rampage in the Seireiden.
Tenmoku Ikko fighting his way to Shana, taking out Fecor (and cutting through Magnesia!) in the process.
Wilhelmina, Rebecca, and Khamsin's fight with Sabrac. He proceeds to take them 3-on-1, Implacable Man-style, wounds Wilhelmina and Rebecca, and No Sells an attempt to beat him the same way as beforenote By removing his body from the ground and destroying it by binding the surroundings to his body.
That scene with Shana going into a rampage while attacking Snake of the Festival Yuji with her trademark Tsundere BAKA! qualifies as this is the first time in this season that she let go of her bottled feelings (since Episode 4) and her stoic facade that she's been wearing all the time.
The final battle between Yuji and Shana (with Sydonay and Margery Daw). Also counts as a Crowning Moment of Heartwarming and Crowning Moment of Funny (for when she just headbutts him from a fit of good ol' teenage drama). At this point both protagonists just throw everything they have at each other, including the spell that can become any spell, Grammatica, and massive bursts of crimson fire.
The climax of the movie — Friagne uses the Trigger Happy Treasure Tool to force Alastor out of his pendant. Needless to say, Alastor gives Friagne hell for almost killing Shana in the process.
Yuji is definitely one of the most cunning characters in the series, if not the most cunning. His crowning moment is his convincing the Snake of the Festival to use Misaki City as the place of Xanadu's creation. His reasoning was that Misaki City was the site of numerous distortions, and that by strengthening the distortions by using Yoshida Kazumi (Kazumi was used by Khamsin in a tuning process to fix the distortions, and could thus be used to reverse the repairs), Misaki City could essentially be turned into a gap through which they could access the Abyss and pour Power of Existence into it, creating Xanadu. While one of Yuji's goals was to create Xanadu and thus end the Forever War, he chose Misaki City as the location precisely because the Denizens that departed for Xanadu would leave behind their Power of Existence, and with this huge amount of leftover power he could then restore all of the humans of Misaki City that were devoured by Crimson Denizens. The Snake of the Festival called Yuji someone who manipulated gods to his own ends, something he fully intended as a compliment.
The final battle of the Second Great War, spanning episodes 18 to 23 of Shakugan no Shana Final, is one long, crazy, fast-paced example of this. It features nine Flame Hazes, led by Shana, taking on the thousands-strong army of Bal Masqué, led by Snake of the FestivalYuji, Sydonay, and Bel Peol.
The three remaining Four Gods of the Earth lay waste to hundreds of Crimson Denizens, proclaiming that they are here to avenge all the humans that have been devoured by Crimson Denizens, and to turn the Battle of Misaki City and its survivors into an eternal lesson for the Crimson Denizens of the future as to what happens when monsters prey upon men.
Kazumi summons Pheles, who retrieves Johann and merges with him to to create a child, Justus. Kazumi joins an eccentric group of Crimson Denizens called Hyakki Yako in protecting what will become Justus, desperately driving through the battleground in a Rinne van before attempting to escape under cover of an illusion. They are joined by Khamsin (who dies protecting them) and later Wilhelmina, when Shaher, the God of Guidance arrives at the end of the battle. Pheles and Johann's creation is so amazing, that an actual Crimson God showed up to take note of it and express her will as to what is to be done with the child.
At the same time of all of this, the Flame Hazes Sale Habichtsburg and Chiara Toscana are zipping through the sky on a vehicle made of light, trying to kill Sale's creator and Mad Scientist Dantalion, who is protected by the Crimson Lord Mammon and a veritable army of giant robots ("Iron Giants") that are also massive bombs. Sale ultimately kills Dantalion by throwing one of his own Iron Giant's into his observation station, while Chiara bisects Mammon with her light "aircraft." The Iron Giants go out of control, and the still-living Mammon dies protecting the tower supposedly creating Xanadu from the now out-of-control Iron Giants, which go off in a massive explosion.
The main focus, of course, is on the fight between Shana, Wilhelmina, and Margery and Snake of the Festival Yuji and Sydonay. Shana calls upon all of her new powerful Unrestricted Methods, creating massive gouts of flame that even overwhelm Snake of the Festival Yuji's anti-flame ring Azure. Yuji fights back with all of his new power and tools, bringing forth the Snake of the Festival's black fire, Blutsauger, and a new spell completely unique to himself, Grammatica, a spell that become any other spell. So dangerous is the new spell that Shana desperately attempts to implement her plan before Yuji can learn to master it. She uses one of her most powerful Unrestricted Methods, True Crimson, which takes the form of a massive fist of fire, to ram the Treasure Tool containing the Xanadu-altering code (which would prevent Denizens from eating humans int the new world) into the World Egg that will become Xanadu. When Bal Masqué attempts to implement the backup spell for Xanadu, the Conloquium, it is revealed that Margery had infiltrated the tower to input the code into the backup spell! And then, Snake of the Festival Yuji reveals that the whole thing was an elaborate decoy meant to delay the Flame Hazes until midnight, when the true plan goes into affect: Snake of the Festival Yuji uses the massive amount of Power of Existence collected by the modified Midnight Lost Child to create Xanadu himself!