User blog comment:B214/Chapter 499: Gray & Juvia/@comment-122.162.151.217-20160829110028/@comment-7418318-20160829110612

The problem with that would be to deceive Erza into believing that's the real Juvia being mind controlled, rather than a Historia.

Because Erza knows that she's just a Historia, then Erza won't hold back I'm sure... if she even needs to fight at all. If she remembers how exactly she did whatever it was that got rid of the other Historias, she can get rid if Histo-Juvia without needing to harm her in the first place.

Neinhart even outright said that his Histoira have no effect on Erza when talking with Irene.