Best things to use early? Four Scorpions, rush right, and Cannister the Node, that will kill it 90% of the time, then mop up resistance. While doing this build another four Scorpions and rush left taking out the Nodes on the way, make sure to mop up the Locusts along each path.
Now after that you'll have a perfect pincer maneuver ready, get a few Cobras ready, don't move them, some Warthogs too, any type.
Use the Scorpions to Cannister the Nodes, take your time on which one to attack , make *** sure you get back into cover. Hopefully, if your not silly, you'll take out all the nodes. Now hopefully all the Nodes are gone, initiate the pincer, Scorpions to the extreme left, and right. When the head gets near, send in a Warthog to 'encourage' the head to turn, they can be fast enough to divert it back to the middle.
While you distract 'His Mightyness', send a Rhino to each side (if they're alive), deploy them as far north as possible to the left and right, ONE EACH SIDE! Having Cobra's in tow with Piercing shot for maximum effect will ensure victory.
I managed to wreck the Scarab on normal within 30-50 seconds of fire with this tactic. Don't concentrate all your forces in one area, if your silly, or the AI is, one stray unit WILL KILL all your forces.
TIP: Move all forces away from your base at the start, move them south, somehow the Scarab sometimes sees them and constantly attacks.
The only thing a Gauss gun cannot solve, is love.