I was a player in a group that finished the original 3.5 version (converted to Pathfinder) a few years ago. We found that the final battle with Illeosa a disappointment. Very anti-climantic. I am currently running a group through it right now I have have restated Illrosa as a Sorcerer. I also bumped her up a level.
I'm not sure restatting and leveling up Ileosa will make much of a difference in the fight. This game is all about action economy and Ileosa is at a disadvantage, even as a sorcerer.
I followed some on-line advice on end-campaign fights, making sure it turned into a battle in stages: you cannot get to the end boss until you take out other challenges first.
So, here's how I did it in a nutshell:
- When the PCs first entered the final chamber, they were mentally accosted by a dream of something close to their hearts, but which ended in a scene that would somehow drain their vigor unless they made a save.
- Ileosa was not alone, but had a "minion" fighting at her side. (In my campaign this was an ex-PC whose mind was taken over by Sermignatto, turning him into an NPC. He even had some extra devils as back-up.)
- The blood pool around Ileosa turned into a "blood dragon", who protected Ileosa from harm. The PCs had to defeat the blood dragon first before they could get to Ileosa, while she could already cast spells at the PCs.
It sounds rather boring when I phrase it like that, but this tactic allowed me to keep Ileosa around for more rounds, making the fiight much more exciting. You can read in detail how the fight went in my journal on these boards.