Okay, let's just look at the facts:
1. CoA raises the attack level to a completely different level. Incomparably with the base.
2. Killer received at least 2 wounds (attacks) from Zoro(with CoA), when as Zoro himself received one single attack(without CoA).
And since the Killer's scythe was intact, it's obvious that he didn't even try to defend himself.
3. Zoro came to his senses after Hiyori’s treatment, when Killer came himself to his senses and roamed around, until he was captured by Quinn's subordinates.
So who is more hardy? The one who came to himself after a stronger attack, or the one who came to himself after treatment for a simple attack. Or is this one Killer stab wound comparable to Zoro’s attack with CoA?
Killer is certainly not the most powerful but do not underestimate his power and exalt the power of Zoro because of this ridiculous hassle.