I'm working on the boss for one of the adventures in my campaign, who is the ghost of a Sorceress. I know that she will keep all of her spells she knew at the time of her death and coming back as a ghost, but I'm wondering how CR works for a single creature that has class levels. If i put her at the same level as the party, would she be too weak? This party is around level 7 average (7,7,6,8,6 for the party's levels). I want this to be a challenging encounter for the party, and I have set up the module so that they will be fully rested when they actually fight her, so they will have all spells open to them.
Would a Level 7 Human Sorcerer(Ghost) be appropriate? I'm assuming that makes it a CR 9 encounter, assuming that class levels on a monster = CR of that monster.
She specializes in Illusion and enchantment rather than being a blaster sorc, mainly because she is based off a beguiler I played in a previous 3.5 campaign, and the guys i'm playing loved that character and the world that this campaign is based in is a future of the previous campaign in which she was horribly murdered by one of the other party members (long story which i should totally post in campaign journals...)
I digress, So would a level 7 ghost be a CR9, and would it be a fair and challenging encounter? Or should I up the level of the ghost, or throw in some extra "minions" so to speak?