Their saves are probably ramped up for HOF.

Yeah to a ridculous level. From what I noticed they would only ever suffer the effects of a spell from reflex, will or fortitude only if they rolled a natural one (regardless of the unit and their inherent weaknesses etc). This effectively ruined HoF cos it limited you to only the one tactic cos their was no way you could rely on them rolling a 1 evertime you tried a spell. In normal mode I always used shroud of fire to kill the treants whilst I danced around them with a monk. In HoF mode I relied upon summons, ranged, area of effect (at least they don't have bloody IE) and twelve paces same as every other fight from chapter 2 onwards!

All in all, I can't really say any RPG has been all that tough. Sure, some were ridiculously long, and I played BGI like a n00b, but most of it is pretty simple. The sole exception to that would be when I installed The Big Picture collection of mods for BGII. Maybe I cling too much to the original game, but I barely began it when I gave up, fighting those spirit trolls in Nalia's keep who hardly give you a chance to fight back.

Well, they're all kinda simple. No big tactics, really. The only thing that matters is how well are your char's stats and spell choices.

 

 

And HoF is like: get yourself a party of half-mages, or half of party being mages and cast instant death spells everywhere, plus use invisibility (if it's unpatched, as someone noted).