The diplomat is located near where you first drop the force field and is not wearing a regular Starfleet uniform. Hostages that are outside the cave and in regular Starfleet uniforms are holograms and can be ignored. In case you missed the diplomat, you can go back and talk to him. The correct location is the south-east force field location (blue circle on the map). If you rescued the 4 in the cave before the diplomat, you will have to beam up to your ship. When you beam back down to continue the mission you will be back in the cave and the boss event will begin. Fight four or so waves of enemies and the boss will appear. Kill him and be done.