It's rather hard to debate when a character can one-shot with haxx since it basically boils down to a first-strike match as those types are typically squishies.
For example, Shiba Tatsuya. He's 7-B
He's rather squishy but with high healing factor, but the problem with him is that he has hypersonic+, potentially relativistic reflexes which pretty much means he's nigh-invincible in his own tier.
His brand of haxx, Demon Right enables him to erase someone from existence by interfering with the target's data in the information dimension which ignores distance, target's movement speed (it is currently unavoidable in-universe) and obstacles provided, he knows of the target's location to look in. Demon Right has no casting time and can also be used on magic spells to outright disable it.
His Elemental Sight also allows him to target and hit 36 targets at the same time, while his limiter is turned o and his Loop Cast System (A program he developed to allow spamming of spells) disabled.
Because of this haxx, he arguably/effectively beats (assuming relativistic reflexes):
Accelerator; tier 7-B, due to Demon Right not having vector.
Uzumaki Naruto; tier 5-C at his strongest, if Tatsuya's relativistic reflexes are accepted which is much higher than Naruto's combat and movement speed, leaving no room for him to even use Kagebunshin. Even then, Elemental Sight can see him pretty easily.
Misaka Mikoto; tier 4-C potentially with Level 6 Shift, because she still can't outspeed Tatsuya's reflexes.
Kurousagi; Tier 3-C with Indra's Spear, because although the spear gives her atleast Galaxy Level power, she's still too slow to do anything about Tatsuya just erasing her before she uses it.
Say what you wish about the show, his boring personality and his annoying brocon imouto, but that Demon Right + Elemental Sight + Relativistic reflexes combo is just broken.
Even with all those things I said, it's pretty much accepted here that one-shot haxx is allowed unless explicitly stated by the thread maker that it isn't.
To answer your question, yes. It's an auto-wi except if the opponent has a means of making a comeback (e.g. Kakine Teitoku and his annoying dark matter beetles that he leaves everywhere for him to spawn from).
