[quote=Tyki]
the zerg could adapt instantly, absorbing their powers. all they had to do was kill natsu and the rest were simple to kill
[/quote]

Gildarts. Destroys anything he touches, capable of wiping out entire islands. Makarov, using Fairy Law, can wipe out anything that isn't an ally. Gajeel, Erza, and Gray will all be preventing you from killing Natsu, which you need to do in order to gain his powers. Furthermore, it's quite likely that the other Guilds would help out Fairy Tail, meaning you might even get to fight more Saint-Mages.