On Xbox 360 this level is a major pain. After following the advice of other walkthroughs I found online I was still getting nowhere. The more time Nod has to develop their weapons the harder it gets to defend your puny little base. The best way to beat this level is to destroy the Nod war factories in the Northeast as soon as possible.
After getting your base in order(check other walkthroughs if not sure how) and your defenses up(especially in the South West corner)start sending out extra predator tanks to take out the Nod war factories in the enemy base to the North East. This is best accomplished by a chain of attacks one after the other with predator tanks in groups of at least four or five. Remember not to leave the base defenses to thin or the base could get overrun. Attacking the west entrance to the nod base seems to work best. Send your tanks directly into the base, take out the laser turret generators and then focus all your fire on the war factories. When the war factories are destroyed they will not be rebuilt leaving you with a whole lot of buildings to take over with engineers.
Once the base in the North East is taken the rest of the mission is pretty easy. Continue to defend against the infantry and helicopter attacks while establishing your captured base to full strength. Don't worry about the convoy thats trapped for now, it will be fine where it is until your ready to safely escort it back to your own base.
Use Nod firepower to take out the base in the Southwest corner and clear the city of any fortified Nod infantry in the buildings. Once this is done escort the MCV back to the GDI base, take out the remaining Nod base in the South and your done.
Send a engineer or saboteur and capture your enemies' construction yard. You will be able to build your enemies' buildings and units. It is also possible for an ion cannon and nuclear missle to operate at the same time.
To play the Scrin campaign, finish both the GDI and Nod campaigns. This will unlock the Scrin campaign.
There is a way to get your mammoth tanks, or any unit for that matter to, at least temporarily detect stealth.
First, you must have an airfield, with at least 1 Orca.
Next, you need the sensor pod upgrade.
Finally, just target the units you want to detect stealth, and drop a pod on em.
The pod will detect stealth, and your unit will react. Simple as that.