How to nerf fortification:
Turn it from an absolute percentage chance into an opposed roll. (
Dev quote: "Granting immunity is bad for the game") For example, instead of Arraetrikos having a 50% chance to negate crits, he could have a +10 AC bonus against crit confirmation. This means that having a higher attack bonus is important for that raid, even if you already could hit the boss on a roll of 2. That'll help solve one problem the devs have faced, where it's hard to keep players
satisfied that their attacks can work, while still building mobs whose AC matters.
Conversely, if you were going to melee a raid boss, you'd want to wear Heavy Fort for something like +20 AC against confirmation, and then try to get an AC high enough so that even if Arraetrikos hits you 95% of the time, he can't always confirm a crit. So if fortification was an opposed roll, player characters would need to care more about both their attack bonus and AC. They'd be less able to say "Meh, it hits on a 2 or higher, there's nothing to be done"
Naturally, Sneak Attacks would have to be confirmed against the same elevated AC, except that they would check the same number as the initial attack roll. And also certain boss fights would need to be rebalanced now that player characters are no longer 100% crit immune.