V3 is done, it's not finished yet but I won't be able to work on it for a while so I decided to upload what I already finished. http://www.garrysmod.org/downloads/?a=view&id=108677

This is the V2 of my kill Breen mission map ( http://www.garrysmod.org/downloads/?a=view&id=103424 ).
This is a map with enemies, not a save.

things I changed:
-added nodes for better npc action; so it's more challenging and more fun
-changed the skybox, it now has more light
-changed the building a little, it looks nicer now
-put Breen in a nice modern heavily guarded office instead of a small brick basement

small walkthrough: when you spawn the door to the building is closed and a helicopter is circling arround the building. The door will only open when the helicopter is destroyed. You will find lots of rpg ammo and health and batteries (dont take it all you will need more at the end of the mission). It takes quite a lot of rpg rounds to take it down but eventually it'll explode.
After that you can enter the building, it has four floors, and each floor has a button. Each button will open a door on top floor. When you used all 4 buttons, the path to Breen will be open.
Breen is guarded by combine elites which shoot combine balls at you and they kill you easily like that. So it's a bad idea to just rush in.

After Breen is dead there will be 6 gunships waiting for you outside, and destroying them is the last part of the mission.

That's it, enjoy