For some of you how are stuck at Graham I have one thing to say to you people before I tell you how to beat him. YOU SUCK! I MEAN HE'S SOOOO FREAKIN' EASY!
Anyway, to defeat Graham, all you need is the Claimith Solias. I forget how to get it. You should be at Lv. 40 AT LEAST! All you have to do is slash at him and avoid his attacks (duh!). Having the Sucubuss soul makes the battle slightly easy.