There actually is. Baam used it during the Floor of Death Arc, and one shot one of Hell Joe's Henchmen who was superior to him. The Black hole sphere has potential to one shot characters stronger than oneself.
congrats, that just puts him above 8 Gigatons by an uncertain amount. Regardless Zoro tanks attacks from those who are stronger than the opponents this Zahard has fought. Occam's Razor and numbers are in his favor here. You Can't assume he's any higher than 8 Gigatons because he one shot someone who's stronger than he is. The best you can assume is that he'd be scaling above 8 Gigatons which I don't doubt. It's the equivalent of me saying Zoro is 30 Gigatons for one shotting Pica.
I read the above, and I'm aware that the 13 gigaton feat belongs to Pica. Whom Zoro one shot with armament haki. Regardless of what he used it for, this is still a form of statistics amplification that he deemed necessary for the battle.
Again this is irrelevant. Zoro uses Busushoku early on so your point is moot.
The tree and your hands analogy, while I see the point, doesn't seem to work as the difference should be more than 1.6x. The above does not answer how Zoro will actually be able to land a hit on Zahard, only that Zahard won't be able to hurt Zoro, which isn't exactly true. Damage accumulates over time, and Zahard's danmaku/ranged attacks, are far better than what Zoro has faced. Kuma's attacks simply don't compared to Zahard's, who is capable of spamming far more of them and at the same time, with homing properties should he choose. And, once again, Cosmos and Second Cosmos can give him some much needed space, as well as aid in racking up damage. In particular with the speed advantage. They both scale to being massively superior to massively hypersonic feats. But, must like the AP, the numbers are what matter the most here. Zoro will be hard pressed to keep up with someone who is 3-4x faster, even with observation haki.
Well for starters we don't do chip damage here so accumulating damage over time isn't true. If Zoro can tank attacks from a stronger character than Zahard then nothing Zahard can do is gonna hurt him. It's like saying that Zahard can now do damage to a High 6-A as long as he has the speed advantage. And while Zahard is faster Zoro is more than capable of keeping up. Granted it'll be difficult to an extent since Zahard does have the speed advantage but you need to keep in mind, anything Zoro does is gonna one shot him. Furthermore I actually need to bring up the Sub-Relativistic Luffy / Foxy thing again, if it gets accepted then Zahard will have no speed advantage. Also Kuma's mouth Lazer covers an insane amount of space and Zoro dodged it while heavily injured.
Though, quick question on observation haki, as it doesn't prevent people like Luffy or Sanji who are better at it from getting hit. Katakuri can see a couple seconds into the future with it. So, how far ahead can Zoro see as his is the worst of the group? One second? Two
This part is kinda confusing. But it's suggested that lower levels of Kenbunshoku Haki just gives a promotion of the future, rather than seeing several seconds into as Katakuri can.
