The user can kill a demons/demonic entities of any form, capable of greatly overwhelming even the powerful demonic entities.
Demonic slayers are so powerful to go toe-to-toe with even the strongest of demons, that many if not all learned to fear the demonic slayers not just because of their incredible power and resistant to all or certain demonic powers and corrupting effects, making them able to withstand them all.
As the son of Vergil, Nero (Devil May Cry series) is a demonic-human hybrid that possesses great demonic powers that allow him to slay the most powerful demons, those who gained a increase in power from the Ascension Ceremony, and even hold his own against Dante.
Dante and Vergil in the DMC universe are powerful demon slayers do to their nephilim heritage.
Exorcist like Dulio Gesulado (Highschool DxD) are specifically trained to slay devils and fallen angels.
Sam and Dean Winchester (Supernatural) have held their ground and even killed demon, something that many hunters before them claimed to be impossible.
Angels like Castiel (Supernatural) are natural demon killers.
Hellboy (Hellboy/BPRD) has single handily killed many demons throughout his years.
The First Blade (Supernatural), when empowered by the Mark of Cain, can kill any demon, including Knights of Hell