StarCraft Walkthrough: 2: Backwater Station
A nearby colony is under attack, and you're going to save it, despite orders to stay put. There are small groups of enemies all around - build up maybe 8 marines, and start roaming. The colony is just north of you - save them and you get more units. Keep cranking out reinforcements, sweep through the rest of the little monsters, then hit their main base in the northwest. By then you should have a fairly good group of 20-30 guys with you to take them out without getting hurt. Your efforts of course are not appreciated. I do like the plot lines in this game.
StarCraft Walkthough Index