User blog comment:Miskos3/Fairy Tail Episode 250: Tartaros arc: Erza vs. Minerva, Review/@comment-24277053-20150912120428/@comment-24277053-20150912160422

Perhaps Jiemma was a master all along, but once he had those 5 strong mages, he had a quo an took over as master during the 7 year time skip.