Tatsuya was not idle either even if Oreki said that he should not involve himself or destroy the Souryuu because Oreki wanted to take kill steal.
Anyway, instead of going on offense as per Oreki's request, he went on defense just like Katsuto and Tsubaki were doing. He's using a magic called Decomposition to make the blood corrosive that was about to hit his allies disappear. To further explain this magic Decomposition, also known as Dissolution or Disintegration, is one of his innate magic abilities that allows him to break down the structural information of any object or Magic Sequence. By identifying a target's design, Tatsuya can decompose physical objects at a molecular level, erasing their structure, or disassembling magic sequences. This ability can be used in various forms, such as projectiles, area-of-effect defenses, or as a "sword" around his body. Notable subtypes include Mist Dispersion, Gram Dispersion, and Material Burst. His brain's focus on this power limits his ability to use other Systematic-Type Magic.
Tatsuya uses his Silver Taurus or to be exact his own made CAD that looks like a gun to cast Decomposition magic. He can cast this magic without CAD but it's more efficient without wasted magic or easy to aim without accidentally making a random bystander disappear from the face of Earth.
Though he tries to help in defense of the others, Tatsuya is mainly defending Erika, Mikihiko, Leo, and Mizuki from the corrosive blood breath by Souryuu, and from time to time he defends Sona's peerage from it. He should be protecting Miyuki but currently, his orders from her are not to help her because she can defend herself and her other two companions. As a matter of fact, he does not really need to make a move but sometimes the excessive corrosive blood slips up from Katsuto Phalanx, he cleans or destroys those to prevent any more damage or if the Souryuu attack a random spot with no one in that, he will use Decomposition to delete to those.
However, Tatsuya sometimes wasn't able to use decomposition to some of the corrosive blood attacks which resulted in the parts of the stadium or arena getting melted. He used Regrowth to restore the melted parts. Regrowth is Tatsuya's innate ability to restore objects or living beings to their original state by rewriting their Eidos. Unlike typical healing magic, Regrowth's effects are permanent, whether used on a human body or machinery. It can even revive individuals with fatal injuries, as long as there's a chance of resuscitation. To do this, Tatsuya reviews the Eidos, experiencing the victim's pain condensed in an instant. Known as "Divine Left," Regrowth can also suppress Miyuki's cooling magic by rewinding the event state.
Tatsuya then turns towards the group of people who are for some odd reason having tea and eating sweets. He had so many questions on how come they have that but he's gonna is Oreki because it's always him, just the like the reason why the Traditionalist and GAU are attacking the 9SC. "No that would be unfair for him because the real reason is that they thought of him as a Yotsuba but in reality, he is not." He said as he erased those previous thoughts.
After thinking about that for a while, Tatsuya noticed that the group he was observing was also looking at him. The group then raised their cup of tea to acknowledge him he nodded once in reply before returning to what he had been doing all this time.
With combined attacks of the Sona's peerage and Tatsuya's group of friends except Masaki, they reduce the size of Souryuu to half of it's size of its original size. However, they are not done yet as they want to shrink its size until they deal a fatal blow to the monster.
---
"So that was the version of Destruction magic of this world," Sona said as she sipped tea made by Oreki for them before they left to do the mission that he had given to them.
"It's called Decomposition if I remember correctly," Kiba added while watching attentively to his newfound girlfriend who is currently fighting and being chummy with Katsuto. Making him quite jealous.
"It's clearly different from the Destruction magic of Rias because she can just destroy things easily without much effort while Decomposition is hard to use or impossible to use if you don't have Elemental Sight," Valerie explained while drinking some corrosive blood that came from Souryuu. Gasper beside her is also drinking the blood but he is clearly intoxicated because he's swaying side to side while moving his feet like a kid. "But luckily Shiba-san had a Elemental Sight" Valerie added.
"And he also a magic called.."
"Regrowth"
Sona was not able to finish the sentence herself as someone finished it for her. The group turned towards the source of the voice and it was Rias who had Akeno beside her.
"Rias, Akeno" Sona greeted them and the group also greeted them. The Duo greeted them back.
Rias turns to look at Tatsuya before saying "So that's the master level of Regrowth"
"Oh right Rias-sama also had a Regrowth magic given to her by Oreki," Kiba said.
"But not on that level yet," Akeno said with a smile "That's why Oreki said that she had to practice so she can be on the level too" She added.
"There's no benefit in practicing it other than to counteract my sword's effect which burns the user to ashes as long as you hold it" Rias shrugged before sitting down together with Sona.
"You don't want to improve your power up?" Sona asked with a frown while thinking that if RIas does not want it why not give it to others?
"Yup! Destruction magic and basic regrowth are all I need," RIas said while thanking Akeno for the tea.
They talk about the things Akeno and Rias did before arriving here which is not pleasant to here as they don't have that kind of hobby like Akeno. So they change subject to mundane things.
They heard shouting from the group who were fighting Souryuu before looking a them. The Souryuu is now 1/4 of its original size and the Fighter group is preparing to deal a fatal blow to the creature and end it now.
RIas' group gave each other a meaningful look before turning their sigh back to the creature.
----
Katsuto seeing this shouted that he was going to use a big Phalanx to squash Souryuu.
Erika who was preparing to jump at Souryuu gave Katsuto a sour look but the latter couldn't see it clearly because she was on the other side of the venue.
"Tsk, I was getting antsy here," Erika said with a hint of anger "He should let me let off steam after waiting to get a hit on this thing" She added while staring viciously at Souryuu. Mizuki calmed Erika down before she said anything hurtful to their Senpai who only thinking the safety of his fellow students.
Erika then turns to the other useless students in this fight "Hey say something." She ordered Leo with a frown.
"I don't need really need to save my ego because I'm just glad that this is over with" Leo replied with a shrug. He then noticed Mikihiko, Erika, and Mizuki looking at him weirdly and in shock. "What?" He question.
"Something is going to happen for sure" Erika replied as she turned to look at Souryuu being surrounded by Phalanx and is about to be squash.
"You shouldn't have told him to say something," Mikihiko said while shaking head.
"Maa, Maa, Maa, It might have happened twice but surely it won't happen again," Mizuki said while sweating bullets and in denial of the Law of Jinx.
"You just added fuel to the fire," Erika said without even looking at Mizuki. The latter covered her mouth as she regretted what she said.
"It..." Before Leo could even finish what he was about to say to invited the Jinx. Something expected happened to the Souryuu.
The Souryuu shrieked like a banshee making the students fighting it covered their ears. The Phalanx surrounding it got canceled after Katsuto lost his concentration. While the students except Rias' group are stunned, Souryuu digs its tail into the concrete all the way to the ground/earth. It then absorbs the nutrients from mother nature resulting in its size to start getting bigger again. The consequence of this action is that the earth started turning black up to where the rooted trees that surrounded the venue were. These trees also got affected by the black soil because all the trees withered and died.
It took a while for the students to recover from their stunned state as they did not notice that Souryuu was getting bigger again.
Shizuku shook her head as she stood up. She also almost fell down again but she was able to regain her balance. She then turns to look at her best friend Honoka. She was about to ask her how is her ear but it seemed like Honoka already recovered from AOE stun because her face looked like she had just seen a ghost. Shizuku followed what Honoka was looking and when she saw what Honoka was seeing, she too gave a horrified expression. They are not the only ones who notice this, as soon as the students recover from being dazed they too give the same expression except for Sona's Peerage who are getting ready to fight again. The Shiba siblings also gave a reaction from it's only frowning.
Katsuto ordered everyone to attack as he wanted to prevent Souryuu from getting bigger again. The students complied and attacked again but this time it was ineffective because Souryuu just kept on growing but to the extent of the surrounding nature. However, they did not stop attacking even for a second and they only stopped attacking after 8 minutes after they exhausted themselves out.
They then gathered together again to formulate a plan to kill this thing while they watched Souryuu keep growing and growing again to maybe recover from its initial size. However, to no avail, they can't think of any plan because Souryuu had an insane healing and it's not just any healing as they notice this sooner thanks to Mizuki's eyes. Souryuu used the life force of the surrounding nature to heal. This is also one of the reasons they don't want to attack any further because nature is also getting affected by their attack.
Miyuki who had been listening to this turned to her Onii-sama and her Onii-sama also looked at her, As if he read her mind, he just nodded and Miyuki smiled in return. What they thought was that Miyuki wanted to use her magic Cocytus to finish the enemy.
She then turns her attention to Souryuu while the students are still planning on what to do now. She raises her arm as she prepares to use the magic however someone gently lowers her arm down.
"Didn't I tell both of you that don't steal my thunder"
Miyuki turned to the source of the voice before shouting "Oreki-san..."
"Hai! Oreki desu"