This post would've been better in SoA, because the battle takes place in SoA (and you'll receive more views and help in there).
As for the Mod's you're using: that tough battle comes with Ascension, while recruiting Irenicus in ToB can be done by Redemption.
Now to your troubles with the final battle. I can't really give any indepth advice, because I need to know your party composition first. What NPC's do you have along? Which class are you playing? etc. Before the battle, I always lay a few well-placed and equally deadly Spike Traps to take out the Black Dragon before he has taken more than one step. Then, I focus on the Beholder, which is a true nuisance with its spells. The Sword and Saravok should be easily killed. Use a lot of Magic Missiles on the Sword (it's like a Mordekainen's Sword, so only spells can kill it), while Sarevok can only be taken out with brute force. Try a buffed up character to soak his damage up (through the use of Stone-, Ironskin, Mirror Images, Protection from Magic Weapons, etc). Like I already said, though, I'd need a party composition before indepth advice can be given.
In the meantime, you could always check out Ascension's ReadMe for hints and tips.
