User blog comment:IamJakuhoRaikoben/Chapter 433 & 434: Ikusatsunagi & Demolition Fist, Review./@comment-4933386-20150512102629/@comment-7418318-20150513214442

Of course in reality Dragon Slayers can'T beat Dragons because in reality neither Dragon Slayers nor Dragons exist.

But in Fairy Tail Dragon Slayers did slay Dragons and quite effectively too. Otherwise they wouldn't have been able to turn the tide of the Dragon Civil War even before Acnologia went out of control.

That the seven Dragon Slayers of the present time weren't far enough along in their training/growth to beat Dragons doesn't proof anything by itself. It can mean they are weaker than their ancient counterparts or it could just mean they weren't yet ready. After all we have no idea on what kind of time-frame the Dragon Civil War happened. It might have lasted half a century for all we weren't told and the ancient Dragon Slayers might have trained under their Dragons for two dacades before being sent into battle.