Actually I think as of now Zeref could defeat Acnologia
The main reason being we saw in the short fight between August and Natsu that even though Natsu was shot with a fire based spell it was so powerful that Natsu couldnt completely eat it or nullify it.
If we apply the same logic to Acnologia...
Would'nt Gajeel die. I mean like he was meant to go to the underworld but because of universe one he was alive. So since it was dispelled he would go back to the underworld rit
