Diff tiers standardization

How can we define low diff and other diff tiers. Take an example, Zoro beats Lucci to one inch from death by a single attack. But his attack was very serious, 3 swords, ACoC, so Zoro was not slacking and kinda know Lucci is too tough that Zoro should push himself to this. So, some say this is mid diff result. other said it is low diff since it is one shot, even if the attacks was serious, it is still one shot and zoro can spam it on a stronger opponent