There has been some confusion about Juvia being vulnerable against steam although she can turn to boiling water herself. I thoroughly enjoyed those two chapters, but I must say this was the one bit I didn’t like. First Mashima had Juvia lose a battle in the water that she would have normally nailed, then fall sick due to her own element in Avatar arc, and now this… (- ︿ -) Anyway, I’m just as baffled about it as everyone, but I’ll try to explain why steam inflicts such damage on her.
First of all, it has become apparent enough thus far, but Juvia doesn’t always alternate between liquid and solid form. When we see her use her water body, it’s because she consciously activates it. This explains why she is immune to most physical attacks when she’s is on guard, but she is affected by said attacks if she’s taken by surprise, like when Raven Tail’s Nullpudding hit her during Hidden, or when Simon did the same at the beginning of ToH arc.
Now, steam works the same way. Juvia is not immune to extremely high temperatures just because she can raise her own temperature to a boiling point. That’s something she also needs to activate. Her normal temperature when she enters her water form is an intermediate one and if she’s hit with a jet of steam, it causes her water body to evaporate at the spot where the steam landed, which leaves her skin scorched when her body regresses to its usual form.
Notice here that Juvia hadn’t realized that she was hit by a steamy fist until the damage was done. She didn’t expect it, and she was taken by surprise.
Also, it’s highly possible that boiling her water body is not something Juvia can easily do. When we saw her boil during her fight with Gray, she was triggered by her fury and immense jealousy towards Lucy. Perhaps she needs to be in a very perturbed state to do that, and that would further explain why the machine guy figured she’s weak against steam.
Still, Mashima could have chosen other things as her weakness instead, such as poison or electricity, to which Juvia has already shown vulnerability. Why he chose to go with steam then? I guess it was in order to save electricity for what we thought would be a fight between the machine guy and Laxus but turned out to be Ichiya comic relief LOL, and besides, a soldier that uses steam would be an easy match-up for Gray, given that they switched opponents afterwards.
This is nothing but my take on this contraddiction, but I hope it clears things up a little xD