In the latest update, several monsters were reworked. Like player characters, a lot of the monster immunities are gone. This should help give players a better chance, but I'm also concerned about making it so that a particular class or feat proves to be unbalancing. Spells, for example, should be effective, but a certain spell should not be effective against virtually everything in the module.
Immunities are only part of the problem. Pugz hasn't replied to my previous post, but Rainbow probably has a strength so high that he cannot miss unless he fails the concealment check against an invisible attacker. Since that check is strength-based, he'll probably make that, too.
So monster ability scores are also involved in the game balance. Monster AC is intentionally limited in the module to make it so that players can hit just about anything with a reasonable chance, but players AC scores (which I tried to keep around 80 max) are much higher. That's caused me to push the monster ability scores higher so that they can hit players.
With such high AB scores, necessary to hit player AC scores of about 100 with buffs, the monsters have very high saving throw bonuses (which are based on ability modifiers). This is unbalancing the game for other players, like casters, who have to beat saving throw bonuses that are modified by ability scores.
I really need to know the maximum player AC achievable and how it's being done. If I can keep the player AC capped to a maximum of about 80, I can then start lowering the monster ability scores to much more reasonable levels. Until then, Rainbow (who is supposed to be nearly impossible as he's like a boss), is going to have to keep his incredible AB.