Difficulty depends on level of damage. Time doesn't dictate it all the time. Brook's comment on Ryuuma vs Zoro fits this narrative perfectly.
If Kaido is blitzed and permanently hurt before Zoro goes out, that's mid diff.
If Kaido is blitzed and permanently hurt before Zoro goes out, that's mid diff.
Kaido fought Big Mom for three days and took no damage, but I would still say that fight was harder for him than a 1v1 with Zoro would be even if he gets damaged more.
I can also think of lots of examples from other comics and manga where a character isn’t taking damage but is clearly struggling.