I've finished up with pre-Tower Ulgoth's Beard. Looking over my last fight (the last one in UB before Durlag's Tower, if you know what I'm talking about), I'm wondering why my weapons weren't effective. All six of my party members were fighting with +2 weapons, or with +2 bows and arrows, or +1 slings with +2 bullets. Why couldn't the best the Sword Coast have to offer smack down these baddies? Did I need specific special weapons or spells or what?

What spoiler? There was no spoiler ;) .
What fight?

The fight with Mendas.

Fire and magic damage (magic missile) works.

There was no spoiler in my message, but I was anticipating spoilers in the reply, hence the warning. ;)

(Spoilers ahead!)
.
.
.
.
.
(Here comes your spoiler, Vedran... ;) )
.
.
.
.
.

Not all the time, however. Dynaheir had trouble punching through loup-garou magic resistance. The wands, however, worked, because even if Kashias and Mendas made their saving throws, Wand of Heavens and Wand of Fire still nailed them for a decent amount of damage.

I'm wondering what weapons can hit the loup-garou. Minsc's Spider's Bane and Khalid's Varscona (Greywolf's long sword +2) were not hitting at all, Imoen's short bow +1 and acid arrows and Jaheira's sling +1 and bullets +2 were hitting every so often, Pirengle's warhammer +2 hit once in a while, and Dynaheir's silver dagger was somehow saving the day.

I didn't want to try Imoen backstabbing with the silver dagger, because she only has triple backstab damage, and it's a pain in the butt to get her in to slice and out of there without major damage.

I'm also wondering if the belladonna I picked up was worth anything. I know my folklore, and belladonna is good for repelling werewolves. Each of my characters carry a sprig. Would things have turned out differently had I not done so?

"Only weapons made of gold can harm the Loup Garou."

If you thought that weapon was useless--nope.

Too late now, of course.

also try the bastard sword +1 (+3 vs shapeshifters) - 2 of them in the game - and the necklace (forget what it's called) that gives bonuses against lycanthropes

The way I defeated them...

SPOILER

Loup Garou are magic resistant, but not especially so. Tank up a character to absorb all of the attacks and pile on the attacking spells until they die.

The Sword of Balduran cut's through the Loup Garu like butter. I think it's +4 against all Lycanthropes.