Not 100% sure how BG determines who gets the kill when multiple people are hitting a target, but I know for a fact people using "shield breaker" set are getting ripped off.
Was playing with a group, the Sorc we were hitting had at least 15K shields when he died, I was using shield breaker I made the final few attacks for the kill, my buddy was just standard DPS crystal fragging or something along that line- and it gave him the kill and me the assist. Happened multiple times, but this instance was clear.