User blog comment:IamJakuhoRaikoben/Chapter 452: The Prelude to the Final Battle, Review./@comment-26822192-20150914170412/@comment-25962249-20150914223130

Put it this way, Zeref is immortal. If Zeref was to fight August, Zeref would deal a fair amount of damage and then run out of magical power. August would have magical power long after Zeref ran out, but Zerefs immortality prevents him from taking any damage.

In short, Zeref can harm him, but August cannot harm him back.