I honestly think August > Zeref, if we remove immortality.Nope , I don't believe Acno in human form can oneshot Zeref even if he wasn't immortal.
Igneel's power gave Natsu the capacity to use FDKM to it's full potential , and I rank Igneel Natsu as Human Igneel. It basically gave him the capacity to dish out attacks on Human Acnologia's level. And those attacks were actually bypassing Zeref's immortality , since it was power of the dead , so it was Zeref's own durability which was tanking those attacks. And we know Igneel Natsu wasn't playing around , he was bloodlusted. Zeref tanked 9 of those attacks and only on the 10th attack he would've been supposedly killed. I don't believe Human Acno can oneshot him. And may be Touka can tank his attacks too for a while
Dragon Acno oneshots Zeref too because Ignia's one roar attack scales to the entire power from Natsu's guild mark which he received from the guild , which oneshotted Zeref.
DF Natsu was a match for Zeref, no guild power bullshit back then. I think human Acno can one shot DF Natsu, I also think DF Natsu < August