User blog comment:Wrath022/Chapter 524: Black Future, Review/@comment-26884814-20170226181338/@comment-153.91.172.162-20170227163143

I'm pretty sure he's talking about raw power. Zeref may have more magical knowledge than Acnologia but it's clear that doesn't mean shit given how Zeref is so afraid of him and why he is so desperate to get fairy heart because he knows he doesn't stand a chance without it.