After unlocking and purchasing Chris' chain-gun (fully upgrade the VZ61) and Sheva's bow (fully upgrade the S75), play the first level and get past your first encounter with the Majini. After jumping out the window you will come to a back alley where you are ambushed by a group of Majini. Kill all the Majini, and not only will you get whichever achievement or trophy you are attempting to unlock, but at the end of the horde you can pick up the Topaz (Marquise) worth 3,000 gold.