2. Build Economy. If you are attacked, make like 3 plants and immediately make barracks
3, Make As Much Troops As Possible. (10 Heavy soldiers and about 5 heavy tanks is enough to kill a command center and undefensive buildings)
Command Center VS Headquarters
Command center is needed and if sold, basically game over. Headquarters, shoot more power laser then command center but costs 100$ more. Headquarters aren't necessary.
Tanks VS Airplanes
Tanks cost less and does more damage but is really slow. If your opponent is running away, get some planes. Preferably space fighters.
Airport VS Space Link
Airport makes planes costs 300. Heavy planes cost 100. Space link makes space fighters costs 440. Space fighters cost 120. Mothership cost 400. NOTE: Space Link is vulnerable to nuke and is destroy just with one nuke. Airport takes two.
I will update this so yah...