There's at least 10 threads already addressing this, please search next time.
As for a strategy, protect just Cargo 2 and leave Cargo 1 to the Spartans. Go mass marines and use the garrison towers. throughout the map. Get those marine upgrades and a couple turrets up on your base and you're golden. Oh and remember, the healing ring heals the ships.
You also may want to try this in co-op, but if you insist on doing it solo, save often.