Besides instant brain destruction, what would it take to kill Tatsuya?

Question

Tatsuya can cast Regrowth on himself as an instinctive and reflexive measure. He did it twice in the anime (the first time when he tuned his sister Miyuki's CAD and enraged the brocon girl by not gawking at her semi-nude body, the second time when he received a lethal blast during the Monolith Code final at the Nine Schools Competition).

The wiki states (probably sourced from the novel) that his Regrowth was fast enough to be imperceptible to everyone watching the match, including Retsu Kudou, the patriarch of the 10 clans (who is probably the most perceptive and experienced magician of Japan, and the guy that could see with bare-eyed the Chinese virus Golden Electron Silkworms injected into Miyuki's CAD).

The time required to cast Regrowth into another subject using his Taurus Silver CAD (confirmed by Miyuki during the third arc) is 0.2 seconds.

Instantaneous brain destruction (like a nuclear blast primary zone, or dipping the head in lava, etc), should be enough to kill him because his magical processor would go down with it.

What else could kill him?

Answer 1

What this comes down to is essentially the following:
Aside from instant brain destruction you mentioned, or instant entire-body destruction (which is essentially the same thing), Tatsuya can repair whatever damage has been dealt to him and survive.

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The only sure way to take him down is to hit him with something that there's definitely no recovery from, such as Ichijou Masaki's Rupture. An attack like this would all but annihilate the body (including the brain, and therefore magic calculation area), meaning Tatsuya would certainly die. However, this is far easier said than done, as we know he is especially skilled at using counter-magic derived from his Decomposition ability.

Having said that, he isn't impervious to assault - he's struck by a sniper's bullet in episode 19 of the anime, and is overwhelmed by Masaki's uncontrolled Air Bullet (albeit while being under limitations) in the Monolith Code final, meaning it is potentially possible to land a lethal attack. And if the right kind of attack does hit, he's just as mortal as the next guy.

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Tatsuya's Regrowth can heal major and minor wounds alike in a fraction of a second, meaning that he can maintain combat effectiveness through almost any injury he sustains. Lacerations, broken bones, and even the removal of one of his arms can be repaired without diverting his full magic calculation area to the matter at hand:

"A thin ray of light was squeezed out and grazed Tatsuya’s right arm.

Despite it only being grazed - Tatsuya’s right arm from the elbow down was carbonized and blown away.

[...]

Into that gun barrel, Tatsuya thrust his gun shaped CAD, the custom Silver Horn, Trident.
With the right arm that was supposed to be blown away.

“That arm!?” Lina shrieked."

_{Volume 11 - Visitor III, Chapter 13}

He does, however, reflexively heal any would-be fatal wounds that occur - and that can be exploited:
When used intentionally, as in the above quote, Tatsuya can control the magic he's wielding. When used reflexively on major wounds, that control is lost and the casting of Regrowth uses his entire magic calculation area, meaning he's left in a vulnerable state during the regeneration process. While only a short period, during this time he's completely open to attacks. Those mentioned in the first half of the answer would have no resistance, and even conventional attacks that strike the brain would be enough to kill him.

This essentially amounts to the point that Tatsuya has only two major weak spots - his brain, and his heart. This is proven during his fight with the Parasite Dolls in volume 12, where he only focuses on protecting those two points from any attacks.

"Amid the incoming superpower [...] attacks coming from all directions, Tatsuya charged forward while only protecting the vital areas of the head and heart and swiftly came into contact with the first Parasite doll.

[...]

Utilizing a full backup of Restoration could block the pain, but under those circumstances, Restoration would temporarily monopolize the entire Magic Calculation Area, crucially delaying any counterattack.

It was precisely for this reason why Tatsuya shielded the vitals that impacted life and death. So long as his Psions were not exhausted, he wouldn’t die even if suffered a fatal injury. However, if he did suffer a life threatening blow, then his survival instincts would automatically initiate a full backup and put every other magic ability on hold.

_{Volume 13 - Steeplechase, Chapter 7}

If you can destroy the brain, that's enough to put him out permanently. If you can severely damage his heart, that would force him into automatic regeneration and therefore potentially provide enough of an opening to perform the first action.

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The only other way to take down Tatsuya is mentioned briefly in that previous quote:

"So long as his Psions were not exhausted, he wouldn’t die even if suffered a fatal injury."

As all magic requires Psions to cast, if his supply was empty, he would be unable to restore himself, reflexively or otherwise. Therefore, if it was possible to completely drain Tatsuya's supply of Psions, a fatal wound (even one not targeting the brain) would be enough to kill him.

Of course, this is practically nearly impossible, as he has such a huge Psion count - even Mayumi Saegusa comments on this after he uses Gram Demolition during the Nine Schools Competition:

"But there are very few people capable of performing this feat. I cannot do it either. Since this isn't an interference ability but a true dispersion, I am unable to create that kind of force with my psion count.

In summation, it's a super heavyweight!

[...]

There were at least ten overlapping Magic Sequences during the incident, but they were all dispelled instantly... Exactly how high is his psion count..?"

_{Volume 4 - Nine Schools Competition II, Chapter 10}

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If nothing else, you could always wait for him to die from old age. While he can repair any injury, he still ages, and will therefore die like anyone else after so long.

Answer 2

Taking consideration of Tatsuya's counter magic and his "Regrowth" execution speed (stated that it is now beyond human eye perception rate - taking perception rate to be 60fps then about 16ms) -

Miyuki's Cocytus: However there is a catch - although it is stated that Miyuki's execution speed of Cocytus is much faster than any other magic she can wield and her execution speed in school practical lab was 235ms (which was actually much slower than her normal execution speed from her specialized CAD) but the actual execution speed of neither Miyuki's Cocytus not Tatsuya's Regrowth has been stated in the series.

Lina's Heavy Metal Burst (Assuming Tatsuya's Regrowth execution speed remains the same): Her execution speed is greater than Miyuki's (except for Miyuki's Cocytus).If her execution speed increases further, then maybe.

Ichijou's Rupture - Nope, will be cancelled before it can be used

Maya's Night - same

Tatsuya's Material Burst - I always thought, what if Tatsuya uses Material Burst at an object near him (or that he is in the range of the blast). He cannot create a proper shield (his other magics have low potency) to completely protect him or has freezing magic

Answer 3

I would like to address this question by first asking if brain damage can kill him.

It is important to understand that the series emphasizes a difference between the brain and the mind. There is also a psion body, and a spectral form (which may be the same thing as a psion body), and the idea of life force.

These concepts are related to the mind, and the body, but the author has raised a distinction between them. The exact distinction is unclear, since mind and brain are often used interchangeably. The mind is also called the "soul" at times. For a Western viewer, the soul is probably closer to what this concept represent than the word "mind".

However, A USNA scientist in the visitor arc suggests that the mind is the thinking device, and the brain is simply a communication device.

What seems to be true, however, is that the brain exists in the physical realm, while the mind/spectral body/psion body kind of don't. The latter 3 also in some senses is an existence different from whatever spells are. Tatsuya can only affect spells and the physical realm.

Let me raise some examples of this. Sakurai Honami died because she used up her psions. Tatsuya was not able to heal her because psion exhaustion is not a physical attribute.

When the female vampire/parasite attacked Leo in the visitor arc, she drained Leo's life force; Mikihiko was able to see this by looking at his spectral body.

The pure force of a parasite is a information body, which I assume is something like a mind. Tatsuya cannot affect this with Mist dispersal because it is not a physical thing.

We can conjecture that magic is done with the mind instead of the brain. I am pretty sure we know that Tatsuya can suffer minor brain injury and still regrowth himself. Furthermore, the pure parasite can cast spells.

We can also assume that death occurs with the death of the mind. I offer this conjecture because Tatsuya cannot revive the deads. I believe the reason for this is that he cannot influence the mind. He can create an identical brain, but without the mind, the person is dead, despite being fine physically.

So we have a question of whether or not total brain destruction is enough to kill off Tatsuya. Since we don't know how long the mind can survive without the brain, we are not sure of this answer.

So, what can kill Tatsuya through regrowth? Ignoring his counter magic, basically that falls under 2 categories.

1) Mental Interference Magic/Other things that affects the Mind

The obvious answer here is Miyuki's Cocytus. Miyuki's magic affects the mind, not the brain, which is how she was able to kill the pure form of the parasite. It will freeze his mind, rendering him incapable of casting further magic. Even if he could cast magic, he couldn't restore his mind.

A less obvious answer is Miya's ability to change the mind. She could simply change the structure of the mind to no longer be able to regrowth. Miya's gate could also temporarily shut off Tatsuya's regrowth, allowing any method to kill off Tatsuya. Presumably, possibly with minor variations of the Pledge spell, Miyuki could also seal away his ability to Regrowth.

In any case, any mental attack could either damage/kill off the mind directly, or control the mind to not use Regrowth, allowing conventional means to kill off Tatsuya.

2) Psion Exhaustion/Uninhabitable enivronment

Dropping him in the middle of the sun will kill him. Even if his mind can revive him once, his regrowth cannot maintain his body in the sun. As such, his brain cannot be regenerated, and his mind will eventually die. If that does not happen, psion exhaustion will eventually set in and kill him.

In short, anything which creates a persistent lethal environment will kill him. Furthermore, if you can force him to cast a self restoration in a persistent lethal environment (say, a fire), he will not be able to defend himself. When his body is healed, it will immediately be destroyed again, forcing his self restoration to activate ad infinitum until he dies of psion exhaustion.

A continuous attack after he is forced to use self restoration does the same thing.

Tl:dr

1)MI Magic 2)Put him in a bonfire, or continue shooting him with a machine gun.

Answer 4

"Can Tatsuya Shiba be killed?" - This question is asked a lot in various forums and more than a few people say "Yes with the use of the Spell Rupture". Except there is a general problem with this answer that no one seems to realize.

He can see the activation sequence and uses Gram Demolition (spell dismantlement). However both Gram Demolition and Rupture have the same flaw of being short range. they can't cast them long distances but Gram Demolition has a slightly wider rage then Rupture due to Tatsuyas monstrous amount of Edios.

Tatsuya can be killed with Meteor Beam as long as he does not see it coming. It’s true activation speed is near the speed of light, but he can hold his own against it (Maya fears Tatsuya because of this fact in addition to the extremely wide range casting of Material Burst) unlike Jumanji Katsuto (next head of the number 10 master clan) and his Phalanx which stands no chance against Meteor Beam, Material Burst, or Lunar Magic.