the talent masochism makes it very easy for a shadowpriest to solo almost every group quest out there.
you don't stay in shadowform, but that's fine. To open, put up power word: shield, throw up devouring plague and shadow word pain on the target, flash heal yourself, throw up vampiric touch.
refresh the dots in between flash healing, renewing and shielding yourself. Masochism will provide enough mana to keep up with all of the expensive flash heals. It can take longer than witha group but you don't need to find said group.
have fun and be safe all.