User blog comment:IamJakuhoRaikoben/Chapter 449: Mavis & Zeref, Review./@comment-203.215.121.60-20150825123224/@comment-7418318-20150825143609

I think Zeref meant her immortality and a possible resistance or immunity to the death waves she shoots out when he loses control of his powers rather than being a genius.

Surely he must have crossed path with several geniuses over his 300+ years of living, but none of them was able to "walk alongside him". Not for long anyway.