Hi,
I picked up Far Cry Classic on GOG a while back. Sadly, my AMD card seems to hate this game with a passion, and I haven't found a solution as of yet. It seems to be lighting-related, as enemies are invisible when not lit but visible when shooting (aka lit by their weapons).
If it helps, here's an annotated video clip.
https://www.youtube.com/watch?v=lOp9r6OPd2M