Well i replayed cologne mission on hard, this times differently.
I used my commando and zone troopers to getrid of all the scrin units and buildings on the available map. Then as my engineers become available. I captured a tib solo and power plant near the bridge. Thus gaining control of sonic emitters facing the scrin base. Then i positioned my troopers and comamndo near scrin base.
As soon as i captured the power plant(objective). All the map become available. I used my commando and troopers to blow up rest of scrin production facility(in two retries). I lost all of them though but it was worth it. The sonic emitters took care of rest of scrin ground units.
Then it was an easy going to build mammoth,rig and pitball, while using artillery to finish scrin defences and buildings. I lost some of my pitballs though to devastators. But it was not much of a resistance.
I do capture the signal transmitter and summoned a mothership, but heck it was pretty useless at that time.