Trolls in icewind dale and character abilities

[NE5]

I have killed the sword spiders but am now coming across trolls that go
down when they are badly hurt but keep getting up again and won't die.

How do I kill these ?

And, I have characters who aren't developing magical skills ie I want
someone else to fire missiles etc at spiders and things. Can I change
my characters attribues or do they improve these skills ?

[Kathy]

if have mage any fire spell will kill them. I get them down and then
have mage use fire on them . if no mage, which is located in upper
corner above store in Kaudihar should be able to pick up spells there,
potions from the gnme in Kuldahar and the temple. Had to restart
becasue I accidently deleted the party I was working with so am just
finishing the dragon's eye 2 level. Those spiders are doozys.

Kathy G