V2 of this map is out (it now has nodes => better npc action, and I also changed some other things. http://www.garrysmod.org/downloads/?a=view&id=108427

this is a kill breen mission I made, (it's a map, not a save game)
it's a big (but very simple) building filled with combine soldiers
it has 4 floors and on every floor you have to find a button which opens one of the doors to breen's hideout
before the entrance to the building opens, you have to destroy the helicopter
and when you kill breen some gunships will spawn outside
think that's it, have fun

oh and if you like this you may also like my other mission, its better, bigger and harder and takes place in an old castle:
http://www.garrysmod.org/downloads/?a=view&id=106193

And my latest mission, takes place in a combine base: http://www.garrysmod.org/downloads/?a=view&id=106544