Well the few possible ways to defeat Achno are
1) use face
2) this is quite farfetched but ... If Achno was trapped in edolas then he would be just some one hand lunatic in cape .... Well only reason I mentioned it is bcoz I would like to see myatogan again :P
3) eclipse gate absorbs all magic...
Well .. He could have been taught by Anna after he became cursed .... Maybe she tried to helpnhim understand the "curse" and how to control it by controlling emotion (??) .... He did interact with various ppl and not killed them b4 he met Mavis .. Like Anna and dragons and quite possibly some...
Anna maybe zeref's teacher as .... 400 yrs ago zeref was a teenager or early twenties (actual age .. ) so .. It can be that Anna taught Zeref a thing or two about magic ... As after all Eclipse gate is COMBINATION of BOTH zeref's magic and celestial magic .... Plus since we are getting close...
I think natsu gajeel Wendy sting and rogue were the last TRUE dragon slayers .... As in they were created for the sole purpose of defeating Achno .... This point can also be reinforced by the fact
1) apart from childhood encounter b/w rogue(raios?) And gajeel , none of other three DS met any...
Well its actually reasonable for them to think achnologia as impossible to defeat as
1) according to myth he IS the strongest dragon slayer/ dragon alive who ANNIHILATED the dragon race
2) During the tartarus arc many ppl (jura , Christina ship of Pegasus) tried to destroy face but were unable...
It could be layla or anna or another Lucy ... Or any female heartfilia .... Coz so far they are called as 'light' or referred to something similar ...
Zeref think Anna as some1 close (1st draganeel x heartfilia?)
Also it place the are referring to maybe a intact inactivated face ... Or OR ...