Yes, he can be Discorded. I have a Disco/Tamer, so i usually use a Frost Mite to tank the Stygian Dragon (Consume Damage helps a lot here) and apply Rune Corruption, then i Discord the Stygian Dragon with a Gwenno's Harp (Reptile Slayer on it). If the Stygian Dragon stays still and focuses on my Frost Mite, then he's easy. If he runs around all over the place Earthquaking though as he chases down guildies, he can be a pain.