User blog comment:KnocknickKnock/Lucy's Unused Power!!!!!!!/@comment-77.194.131.128-20140715085522/@comment-7418318-20140715095434

Except for a few select individuals, Fairy Tail has the strongest Mages in Fiore and always fights the strongest foes available.

Strong potential training partners and strong enemies. Sounds like the perfect enviroment to become stronger. Lucy has gained more power in this half or so year of being a Fairy Tail member than she did in the entire rest of her life, which includes a full year of traveling alone after running from home.

So I don't quite understand why her leaving the guild would help her get stronger.