User blog comment:IamJakuhoRaikoben/Chapter 452: The Prelude to the Final Battle, Review./@comment-26389992-20150914221641/@comment-7418318-20150915073701

In Jellal's GMG fight, don't forget that he was trying hard not to let his true identity slip, which greatly impeded his fighting ability.

The moment he intended to use his true power, the battle was cut short by Ultear, so we really can't judge his true potential power.

But that he was able to so seamlessly copy Mystogan's magic in such a short time speaks volumes about his ability to learn new spells, further cementing what we learned from the flashback when Brain taught him Self Destruction Magic in the flashback.