Trying to get through the Challenges (at Gold), "Opening Gambit" is proving annoying :/ Best is 3:30~ of so, need 4:00 or better for Gold. Even more annoying is that it's at "Normal" speed.
I just managed this one, albeit with only about 15 seconds to spare. Took me a few goes though... Anyway, what I did was:
Build up to 11 SUVs to max out the supply (all mining minerals)
When the 11th SUV is built, send one to build a barracks, and another to build 2 supply depots. When the SUV building the barracks is done, send him down the ramp to build a bunker. 4 marines go in to see off the first wave (the bunker needs repairs though).
Then I just built a second command post, and just built loads of SUVs. I maxed out the first base, but found I didn't need a full compliment of SUVs at the second. I SUVs started on the gas once my second command post was up (3 SUVs each on the first base), and built refinaries at the second once there were 5 or 6 SUVs on minerals.
As for building, I figured out that to finish it with 4 mins left I needed:
* 2 factories, to start outputting with 7 mins left
* 2 tech-barracks, to start outputting at 6:40 left
... and another couple of barracks for marines towards the end (plenty of minerals then, just short on gas).
Anyway, I went for the factories a little earlier so I could have 4 siege tanks (in siege mode) by the time the first 'big' wave comes, with ultralisks. I started pushing out ghosts on 7 mins, and scrambled marines to the finish line.
But yeah, things like this put me off playing online. I'd have no chance doing this kind of optimisation 'on the fly' at faster game speeds, and adapting my strategy based on what the enemy is up to. I'm all well and good at planning, but when the game speed goes up I just fail badly and important things get forgotten.