User blog comment:FairyRaven/Fairy Tail Chapter 407: All for My Demise, Review and Commentary/@comment-203.111.234.105-20141102003342/@comment-7418318-20141102144109

I wouldn't count Sting and Rogue, since that was pretty much assisted suicide rather than defeating a foe.

Maybe no living and still Human Dragon Slayer has defeated a Dragon, but we got a first hand report from Zirconis about Dragon Slayers in the past - not least of all Acnologia - killing Dragons, so they are "real" slayers. Immature and/or with too little training to do their job, but still the real thing.