Enma is one of the 21 O Wazamono grade swords. It was once wielded by Kozuki Oden alongside his other sword, Ame no Habakiri, and is one of the two only weapons known to have ever injured Kaido.
As stated, drains excessive amounts of Haki from the user. Enma does not give the user power they do not have.
Zoro as seen in Dressrosa was slicing up Pica’s golem body with his haki clad swords. This by default shows Zoro can already cut that cliff in Wano if he uses haki and any sword he wants.
The difference between these two situations is that Zoro WANTED to cut Pica and he did, while in Wano Zoro WANTED to just cut a tree and he ended up cutting a whole Cliff. This right here is what makes Zoro a failure as a swordsman in this sense.
During Alabasta we get a very clear breakdown of what a good sword and and good swordsman is. When a swordsman makes his sword only cut what he wants to cut is when they, the swordsman and the sword, are actually good.
This is demonstrated on numerous ocassions.
Zoro gains the abilty to cut steel WHEN he tells Wado Ichimonji to not cut the leaves and the sword doesn’t. The same way his master failed to cut a paper because he chose to not cut the paper.
Zoro argues with Sandai Kitetsu when it cuts too deeply into people the same way he argues with Enma for drawing out too much of his energy
Zoro uses Shusui for the first time and draws out too much power and fails to properly execute an attack he learned in Skypiea and then comments that he has to master the sword which is similar to what he said about getting used to Enma.
Swordsmanship follows intent. Zoro isn’t a better swordsman for UNWILLINGLY cutting a cliff WHICH HE COULD ALREADY DO. He is a worse swordsman for letting his sword do what he didn’t want to do. Or at the very least, he’s just not a good enough swordsman to use this particular sword.
Therefore when Zoro trains himself to become strong enough to STOP THE SWORD from taking his power unwillingly, the entire Haki draining thing will cease to exist. Enma will function exactly the same way as every other sword Zoro has, the same way it did exactly the same amount of damage as Ame no Habakiri to Kaido when Oden used it.
*Theory by ImmaIvanoM