Wow I'm surprised you fouled up 2 pairs of fiskars - I used the 9" orange handle ones (dressmaker model I believe) on 12oz biax without any problems (even after there was epoxy on the handle and outside surfaces of the blades)
BB has a great tip on method by drawing back as you snip
I just bought these cordless scissors off of Ebay for < $25.00. Quite frankly, I haven't tried them on fiberglass yet--in the reviews they say they work well --but they did go through heavy carpet and all 8000 Amazon boxes my wife ordered for Xmas like a hot knife through butter.
I bought a pair of fisker amplify fabric scissors for around $25 and worked great. I've used cheap scissors before to cut fiberglass cloth and was pleasantly surprised how much easier it is with a nicer sharper pair.