Have these büggers been fixed in the fan patches? I mean, DR 15/-- is... INSANE. If not, are there any plans of reducing them to a "believeable level" (i.e. 15/+2 or something)?
As most of the people modding seem to be of the opinion that the game isn't very hard, I doubt there's much effort going into making enemies easier. There may be a way to do it yourself, though, by editing the appropriate entry(ies) in one of the game files. Try poking around in the modding section of this site. Until then, flank for sneak attack damage, and use cleavers to get lots of big criticals.
The Galeb Duhr can be beaten by using fireball spells, especially if the caster is 9th or 10th level, or so I have found. Just make sure you have one fighter in the party to hold them off while the Galeb Duhr congregate behind to get at the party. Then fireball them a few times. A fireball wand is good here.
Cone of Cold baby Shise, this is what the whole fun of wizards is about! I went in wiz first, cast a web to trap 'em, then a couple cone of colds to bring them down to size. My ranger and barbarian picked off any stragglers - and they could mop up the stuck ones with their freedom of movement rings. I believe web also decreases a character's AC? At least they seem easier to hit...
I guess the way i handled them is kind of cheating but anyway i made a amulet of mighty fists +20 for my monk and just had him pummel the Galeb Duhrs with power attacks. And i had my sorcerer use the wand of chain lightning to weaking the Galeb Duhrs that my monk wasn't working on. Chain lightning works very well and you really should have that wand by now. They all went down pretty fast for me since my monk did 30-40 damage each hit with that "special" amulet
They're beatable, but they're definitely a lot harder than they should be by the books and certainly don't give out XP appropriate to how tough they are.