You can be either the Wiji Waji or not. If so, seek everyone and kill them melee. If you are not, hide until Artifacts appear, which are yellow crystals. If you get them all, a gun will appear somewhere on the map, with which you can kill the Wiji Waji and save all the survivors. If you don't manage to kill the Wiji Waji before the time runs out, everyone will die.