I definitely agree that playing on tactician should be a challenge but Larian is known for braindead AI that's easily confused by elevation and line of sight. Or just cheesed with explosive barrels and invisibility. Would be great if AI tried to cheese the system the same way players do instead of just getting HP, AC, DC, and save roll buffs. There are a few enemies that go invisible after attacking you but they all just stand in the same spot so you know exactly where to throw an aoe to reveal them. And that's pretty much the best an AI can do.

Some suggestions would be...
-Have enemies break line of sight after attacking the player and try to end their turn in a spot that the player would have a hard time getting to. The game does a half decent job calculating an optimal path between 2 points so this shouldn't be difficult.
-Have enemies shroud their weak members (low hp archers, concentrating casters) with things like smoke arrows, darkness, fog. And when it's their turn they can move out of the cloud, attack the player, then go back in which will prevent them from being easily picked off with spells and ranged attacks.
-Give enemies a level of wizard or sorc. Having more enemies that can have the shield spell up for a few turns will help them survive MM volleys and being dogpiled on.