We’ve been through Fishman Island, Punk Hazard, Dressrosa, Zou, Whole Cake Island and now Wano… but we have never seen Zoro taken to the limit like Luffy has with Doflamingo, Cracker and Katakuri.
This means that we don’t know how strong Zoro actually is so.. let’s look at it like this: for 2 years Zoro was trained by “The World’s Strongest Swordsman” Dracule Mihawk. Zoro not only learned but mastered how to use Armament Haki but that would not be the only thing he learned.
During the Marineford War Arc when Mihawk wanted to join the fight Boa Hancock stated: “They say the most skilled fighters are struck by fear and are left unable to move when they see those eyes.”
Demons and Spirits play a huge roles when it comes to Top Tier Swordsmen like Zoro’s ability to summon Asura.
Note: Zoro’s eyes glow red when doing illusions
Like I said before the opponents are struck by fear and are left unable to move. This is not Conqueror’s Haki, it’s a Demon based ability which allows the user to create illusions.
Zoro’s Asura being an illusion means that he doesn’t actually have 6 arms and 3 heads, it’s just an illusion which enhances:
Note: this was pre-timeskip.
Other time was the one Hancock was talking about Paralisis Inducement Zoro demonstrated this in his fight with Monet.