That's why it's nasty to fight fundamentalists: They don't care if they themselves get killed in the process as long as the target dies as well. I have to agree on it being an asspull in the sense it could link to Gray, whom the Grimoire mage had never even met or seen, just like that. On a general level it would be pretty weird and very much cheating if she could simply seek out any weak member of the enemy group, link that person to the boss of the group (since the weakling might respect the boss a lot to fulfill the spell requirement), and then killing the weakling would result in the death of the boss as well. Either she's simply lying to Juvia (unlikely seeing how this is so stupidly honest series) to gain an upperhand or there's some other weakness.

But then again, this is such a superficial story that it's really no use to analyse anything in it.

And I really hope Juvia will go back to the other hair/clothing style... I only really started to like her when she adopted it in the first place.