Zeref got infinite magic and control over space and time. However, I don't have any reason to believe this made him incredibly more powerful than he was. Natsu was simply able to let off a bigger attack than Zeref could in their encounter. Basically Natsu's attack potency is greater than Zerefs. Zeref did not even become more durable. We saw this when he was hit and obliterated the first time. That wouldve been impossible if Zeref had been significantly boosted. And in retrospect, that last attack simply meant Natsu damage output is greater than Zerefs. Since Zerefs durability did not increase, and neither did his attack potency, he could be defeated if he got hit by attacks strong enough to overcome his durability and endurance. It isn't that farfetched TBH, granted the fight wasn't exactly sensible, but it isn't an absolute impossibility.
Basically, what I'm saying is that Natsu seems to have the highest attack potency after acnologia (maybe August is in there too). But then, things would have gone differently if Zeref hadn't attempted to go head to head with Natsu at the last moment. I have no reason to believe either Natsu or Zeref are beasts in CQC. At least not to unnatural levels where they can't be matched. And neither of then will easily obliterate enemies in a battle Imo. They both have KO attacks, but so do a lot of characters. Heck the guild was able to withstand these two fighting for extensive periods of time. So Gildarts is strong enough to fight with Natsu and withstand majority of His attacks. We saw Zeref punch a hole through Natsu after all..