So happy I finally killed Corvus. :3
I think his page is wrong- you don't need to be almost level 60 to defeat him in his demonic form. My Warrior, Minstrel and Martial Artist were lv. 45 and my Priest lv. 40, and not one of my party members were dead when I defeated him. Admittedly, I had to use a Yggdrasil leaf on my Minstrel, but it doesn't matter.
My strategy- Martial Artist Psyche up to 50, Warrior and Minstrel use Gigaslash. Priest just heal and attack.
I don't bother getting Martial Artist to Psyche Up to 100 because he knows Distruptive Wave, so it isn't worth it. 50 works just as well.