Nah, people will wind up HATING unfair AI. if you're going to use unfair AI, save it for boss characters. You WANT people to hate boss characters. On top of that, if the player has been cruising through the regular fights, and suddenly encounter a boss monster with evil AI which forces them to change tactics, I think you can call that a result.

After the boss fight, add a few of his evil tricks to normal fights afterwards so that the player can't go right back to their complacent ways.

The AI in Serinor is, at the moment, somewhat limited by the hate system. However, the hate system has no bearing whatsoever on what action the monster initially selects; it just affects who the monster ends up selecting as a target. And, ideally, hate will only limit actions that actually do damage. Abilities such as silence and stun may get thrown around at characters who are simply annoying them as opposed to those they actually want dead.

Additionally, some overseer type monsters (who would still be common enemies) could have the ability to direct their minions to attack a certain character. I think this was done in one of the Breath of Fires. So some monsters would be capable of affecting their comrades' hate as well, not limiting this ability to the party. I included abilities like 'Guard' and 'Guard All,' and I really kind of think it pointless not to include situations where they'll be useful if not necessary.

Done and done. Hate calculations already actually have a bit of blur built in around the edges, so I even know exactly how big to make the gap.