This is my take on FH Zeref and Acnologia. Let me know what you guys think.
What has FH Zeref done that puts him on Acno level? That he managed to "kill" Natsu after he completely blew off his magic power and was exhausted? This is way less impressive than weakened Animus "killing" Natsu, while he was very much energetic. Human Acno managed to one shot full power Animus back in the days, by the way.
After Natsu got revived, he managed to hold his own against FH Zeref BEFORE "savage flames" showed up. Sure, I'll agree that we can't claim this is regular/base Natsu, since Natsu was amped on POF this time around. But, Natsu used savage flames only in the final attack, everything up until then was regular POF for Natsu.
As for dura/defence. Animus tanked multiple attacks from one of Natsu's strongest DF. And after all that onslaught, he still was in one piece. FH Zeref got vapourized by DF Natsu's (a weaker version compared to what Animus faced) full power attack.
Now, if human Acno can one shot Animus. And Animus can take full onslaught from one of Natsu's strongest DF while being in one piece, while FH Zeref is vapourized by full power attack of a weaker DF Natsu, what does it say about Zeref? It suggests to me that, if Zeref wasn't immortal, time magic or FH or not, he would've died right there. It's immortality combined with FH & time magic, that allowed him to revive right afterwards.
I see claims that savage flames Natsu is > DF Natsu, including the Ignia/AF/Igneel DF mode and the one used against Merc. This isn't true.
Savage flames Natsu has flames HOTTER than regular DF Natsu, though if we're speaking of special cases of DF (like ones used against Animus, Merc and Aldoron) that's not necessarily true either (the one against Ignia for eg, burned Natsu's skin the same way savage flames burned Natsu's skin, but this was an year later and hence more impressive).
What savage flames did was, burn FH Zeref's time magic, so that he couldn't just turn back his own time like he did earlier against DF. This is because of the nature of that magic and heat, rather than pure attack power.
Savage Flames Natsu's full power attack only did enough damage to knock out Zeref. Injure him to the point he couldn't get up. But he was still in one piece. DF Natsu's full power attack completely vapourized FH Zeref. This tells us that, on pure attack power, regular DF > savage flames Natsu.
Now, I do agree that FH does make Zeref jump up by tiers. Fairy Heart's effect on Zeref can be sort of compared to Grimoire Heart's effect on Hades. Grimoire Heart was Hade recreating Fairy Heart, since he's the one who created FH to begin with. GH, like FH, is a practically infinite supply of magic power, tho GH needs life support to be kept working.
Like FH allowed Zeref to access time lapse, GH allowed Hades to access devil's eye. Upon activating GH/Devil's eye, FT members saw Hade's MP jumping up tiers. We can expect the same for Zeref as well. Since base Zeref was easily overwhelming regular/base Natsu; and regular/base Nastu dealing with FH Zeref in combat pretty fine post revival, this does mean that the POF he was on after revival was quite significant.
If we're talking about portrayal, yes, there are claims in the manga which suggests he can be a threat to Acnologia. Other than confirmed God tiers (Igneel, God dragons), FH Zeref is the only character to even remotely have any portrayal of being a threat to Acnologia. But those are from Spriggans, and it was more of a hope than expectation. And like Axiomous said, none of the Spriggans knew that much about Acnologia - that he was the magic dragon, can eat all magic and was near immune to magic. I'm not saying to ignore this portrayal completely, but it is to be taken with a grain of salt. Combining it with FH Zeref's feats, I see no reason why Acno wouldn't stomp in this fight,
Natsu was able to deal with Zeref's time reverse shenanigans by burning his time magic. Acnologia can accomplish the same by eating his time magic. Once time magic is out of the picture, FH Zeref is nothing to Acno. Acno can just knock out Zeref quite easily, pretty sure Acno can casually create attacks stronger than regular DF Natsu's full power.
