The sword you get early in the game +1, +3vs shapeshifters is bugged I believe. I went to the druid grove and fought the shapeshifter there in order to get her head for the Dao Geni. However, even while weilding it, I couldnt hit the shapeshifter but after I switched to my +2 sword, I was able to hit her fine.
Anyone else notice this?

Maestro

I dont know if this will be much help. I am not sure how it works in the game but when they use the term shapeshifters they usually mean lycanthropes (were critters and so on) and the like or maybe even dopplegangers. The lady you went after was a Rakshasha. They are not shapeshifters they use magic to change the way they look. If they are going by the D&D rules then the +1 sword will not be affective against them. If they go by the rules and so far from what I have seen in BG and BGII they stick them pretty well. Anyway I hope I have helped.