I'm having trouble getting past Silver on Opening Gambit, the challenge mission where you must produce 30 Marines, 8 Siege Tanks, and 8 Ghosts within 14 minutes (for Gold). The best I've been able to do so far is to finish with 2:23 on the timer, which is 1:37 short of Gold.
Typically I haven't had issues holding off the attackers, but simply can't get QUITE all my Ghosts or tanks out in time. 1:37 short comes out to about 3 Ghosts late.
I think my issue revolves around timing my expansion compared to moving to the more-expensive Ghosts and tanks. My strategy so far has been to wall off my base as suggested, and get two bunkers down: 1 up on the cliff, and 1 down below. Usually I'm expanding as the second bunker is going into production.
Siege tanks stay up on the cliff when they start being produced. My best attempt so far had 2 Factories+Tech producing tanks, 2 Barracks+Tech producing Ghosts, and 1 Barracks+Reactor producing Marines, but I was still too late getting production kicked off.
Do I need to expand earlier? If so, when should I do that relative to expanding to gas and working on tanks and Ghosts? Should I crank out all my Marines before moving to Ghosts?