Computer is rather boring, do what Them said, and tower, then when you attack, hit buildings and their peons will come to you, its a flaw w/ the AI and quite boring. You should be able to consistantly beat the computer on hardest settings by yourself. 5v1.
I only use the computer to practice timing, most skills developed fighting the computer wont help you in multiplayer games (and notice I said most, and fighting not talking about developing economy).
He who has the last peon standing, wins!