Note: do NOT attach your artillery to the CB yet.
Send in a couple trucks... like, 3 (set to "do or die") through the center of the map; we are going to use it as "bait" to attract the attention of Nexus artillery.
Move them up the ramp, and have them build a couple fixed-emplacement artillery pits IN FRONT OF your artillery brigade (we want the first ouple enemy volleys to NOT hit our mobile units ;p). As soon as they are done being built, they will fire, and enemy artillery will focus on them; THEN attach your own artillery to the CB.
Nexus has 2 Angel Batteries in the NW base, and 2 in the SW base. Once all 4 are down, your Artillery brigade will be safe (unless NExus airpower gets agitated by them, but I've yet to see that happen). So re-attach your artillery to the nearest sensor, and let it do work for you

.
Like... oh, say, here:
(Wait for the one further SE to kill the forwad Nexus towers before setting up the one further NW).
Your MBG shouldn't be necessary for anything in the NW base (save for Cleanup).
After the visible towers (3 railguns) and units (a couple cyborgs) have been handled, go ahead and send in your MBG through the valley, heading north, and into the base to finish it up.
You may run "into" a few nexus cyborgs during your approach (they are positioned near the W edge of the map, about halfway up), and a "base" (that is nothing more than 2 defenses and a sensor). Skirmish them if desired; but if any of your tanks try to retreat due to damage, make SURE you take control of them and send them around the north to the repair bay. If they try and go south then east, the Nexus base defenses will obliterate them. Or, set up a temporary repairbay in the depression at the base of the ramp coming from the NE base... but beware of nexus attacks.
The Nexus rocket towers at the S base have a HUGE range (they are known as Scourge, you'll get them eventually... they aren't even classified as Rockets, but rather Missles), so be careful. Odds are, if you fight the "base" in the middle of the map, they will fire at you. Thankfully, unlike when this game was first released, they are no longer death to anything they hit, but still... be careful of approaching the S base for now.
After you have "cleaned up" the NW base, it's time to deal with the Annoying Nexus Airforce / Strike Squads (AnAss). (while building all this, have your MBG kill the trees on the western ledgge so that when they press south they have a clear path... but again, be carefuly of Nexus Scourge towers).
Yes, those are Rotary MG bunkers in that screenshot. They belong there, trust me; not for this stage, but eventually. And all those AA guns are NOT 1-shot wonders; they too will get use in later stages.
So, you want to get into the SW base, but not take horrific counter fire...
Artillery is a good method. And I prefer this:
You'll notice where my sensor tower is placed (in that nice fuschia circle)? That is as close as you can get without the visible Scourge towers firing. It is, however, close enough that your own artillery can begin it's bombardment. From here, you can get the eastern Scourge tower, 3 Railgun hardpoints, and a couple SAM sites. This should be enough of a "hole" in the NE defenses of the SW base to allow you to begin pushing into it with your MBG.
Set your MBG to HOLD FIRE. This is DEFINITELY a stage we want to milk, wich means we need to leave something standing.