1) 4 Turrets...2 with anti infantry and 2 with anti vehicle.
2) Build out supply pads, upgrade them and an Armory.
3) While that's building take your tank and forge to kill the two nodes on the left carefully and then take the top left node without losing the tank or forge.
4) Use the marines to gather supply points at base on to the left.
5) While your doing that upgrade population, the unit build time thingy, and turrets all the way.
6) Destroy the Armory and build another supply pad and upgrade.
7) Also be upgrading wolverines all the way and start pumping them out.
8) Leave the tank at your base with Forge and then take 5-6 wolverines to kill the nodes on the right side while avoiding conflicts.
9) Start attacking scarab by the 12 min mark with 10-12 wolverines making sure to use Y every time possible and mac blast.
10) Pump out another tank or two and then upgrade them and leave them at the base to defend. Regen as needed.
11) Keep moving your wolverines back and forth at the scarab to avoid losing them. Only move 4 at a time to prevent jamming them up.
12) At this point you simply keep regenerating your tanks and turrets while doing the side to side thing on the scarab paying close attention while using Y and mac blast whenever possible.
This will continue until the 17-18 min mark and then you are done. This will maximize your combat bonus and most importantly your time bonus finishing by 17-18 minutes. The key is to not lose any units when clearing the nodes and to start on the scarab ASAP @ around 10-12 min.
I did this with all skulls on and barely missed the mark but then I tried the same thing with NO skulls and beat it on legendary with GOLD easily.
Micro, Micro, Micro / Regen, Regen, Regen / Y, Y, Y / MAC, MAC, MAC