Firstly, I watched One Piece fully (until the latest episode) while Fairy Tail only the part until pretty much before Fairy Tail Zero and then I stopped due to some reason, so Idk if Erza learned/got new power in the recent episode.
Secondly, about One Piece and Fairy Tail, It’s totally different world, different power, different stats or different rules (Logic). What I mean by that is that, In Fairy Tail as all of us know, each of the characters strength are based on magic powers (primarily) and they will get more stronger as they train.
While in One Piece, there are Devil Fruit powers which is kinda awesome if you get a good power from eating the unknown type fruit and Power based on training just like Zoro (Strength).
So based on my own personal thought and reasoning, If Erza do fight with Zoro, I thinkZorro will win. Even though Erza have different equip abilities, I think it will take a toll in her body to keep changing it. Zorro in the other hand are fully based on Strentgh and Speed. His 3 sword can have many combination, If he try to cut something hard he will use his 1 sword to concentrate the power and if somehow it’s kinda multiple attack / wide range attack, he use all three to form combination to counter it.
Erza power sure have good quality, she is very determined and trained since she was still a kid but I think it’s not enough to beat Zoro. Other than that, Zoro have several Haki which is kinda amongst the higher league in One Piece.
It’s more to Quality (Zoro) rather than Quantity (Erza).
Look at this cool Zoro.