Some people say that it’s unrealistic for Erza to arrive in Grey vs Natsu fight before Juvia and Lucy, I beg to differ. Let us examine the geography:
First, in chapter 494 (Spriggan 12 united) we see a general location of the guild:
Second, we see in 503 (Natsu turning into END and Grey encounters him) that Natsu is moving towards somewhere with soldiers. Remember, Brandish brought Lucy & co to somewhere isolated, NO SOLDIERS PRESENTED, but clearly we see that Natsu traveled at least far enough to encounter many soldiers on the way.
Third, it is clear now that Natsu arrived IN THE FRONT OF THE MOUNTAIN/GUILD
Finally, I conclude that the location of the key characters are such:
The reason I put Juvia and Lucy together is because I have no idea which side they could be on. They are on the sides because there were no soldiers present in their way and we all know that soldiers were gathered in the center. Also, they can’t be at the back of the guild because there are distinguishable huge mountains that were not present in the Brandish/Dimaria fight and the same goes for Invel v Grey&Juvia.
It is completely reasonable that once most soldiers were eliminated, Erza could see Natsu vs Grey clearly and rushed there to stop the fighting. Juvia is moving slow because she’s still seriously injured while Lucy and Happy don’t seem to be close to Makarov since they don’t know where the light came from (they are looking at different directions after Fairy Law, whereas Mira who’s close clearly knows where the light is from).
I think Juvia and Lucy are kind of close but Erza was hit by Makarov’s death and rushed in to stop the seemingly unreasonable fight between Grey and Natsu (come on Erza doesn’t even know about Natsu’s situation).