When they said that the emperor had 11 mages equal to Serena,that was a figure of speech,since it's quite clear there is an hierarchy. That Brandish is number 12,so she is technically the weakest.
I imagine that the stronger ones are Hades tier.
One observation:In a fight,strength isn't enough. Ultear could melt Gray's ice and she still lost. Zancrow could eat Natsu's flame and ,despite what some could perhaps rightfully consider an asspull,Natsu won. Natsu could eat Jackal's explosions. All fights are also determined by how can the opponent negate or weaken the other's magics. It isn't absolute as was shown by the first two examples I said,but it's still there. For instance,that guy could negate Lucy,Erza and Mest's spatial magic. Not even Erza,who is among Ishgar's strongest legal mages could get around it.
Maybe Brandish has a type of magic that allows her to get around or overpower Natsu and Gray's. Or she is simply that much stronger. Honestly,if Natsu and company couldn't one shot Avatar's side branches,after defeating the mightiest of the dark guilds in Ishgar,I would have been worried,lol
Also,people bitching about Natsu are being idiots. He is the protagonist. If you hate him that much,you are wasting your time with the manga. Plus,an invincible hero would be perhaps Naruto,Ichigo and Luffy. Natsu has a terrible record. Many of his victories needed support,and he lost many times because he tends to pick opponents that are clearly stronger than him or can get around his magic.