If it's Haven's Fall you're talking about, I can sympathise. Played from 3am to 6:30am this morning and I'm just starting to feel I might gain the upper hand. What I'm doing is focussing on a settlement at a time, obliterate it and pull back to save my forces. Roaming around the map with an army will only see them flattened very quickly.
Make sure you send medics and SCV's with your troops for constant healing goodness. Make sure your base is bullet/bomb/nuclear proof as to not distract your attention with irritating micromanagement every attack on your base, leaving you solely to focus on taking on the objective.
I know it's hard (I play on Brutal, it's almost impossible at times), but just a bit of perseverance will get you there.
. I think he tech'd up too quickly, whilst I concentrated on lots of smaller units. I made a lot of mistakes though! Hmm, a few more practice matches methinks until I sort out those inititial build issues.
