Left click any creature several times to make it explode.
Units hidden dialogue
Keep left clicking units to hear secret dialogues.
Piercing the Shroud Mission
To unlock the secret mission, on the "Media Blitz" mission, where you get to control the Odin in the city, head down to the far bottom right corner of the map. There's a civilian building on a little floating platform. When destroyed it will reveal "Secret documents". Pick these up with any ground unit, and then complete the mission as normal. After, you will now have the secret mission available for selection from the Star Map.