When you attack you should bring some ghosts. There attacks are useless, but there lockdown and cloaks can come very handy. If you are attacking Terran, and going to there entrance. Then you notice they have 6 battlecruisers with some tanks, bunkers, and turrents. First I would just bring in some marines and wraith and attack the defense. While the enemy is busy, just move in your tanks and seige them all. There defense is now gone, well, not quite yet, they still have the battlecruisers. Just bring in some cloaked ghost and lockdown all of them. Now, you should have some backup anti-air or some wraiths, goliath, marines, or battlecruisers too. Now just go attack their battlecruisers, with your backup. Now, just let your seige tanks do the rest of the building killing. If they have any lift off buildings just attack with your backup force. This strategy helps me in several games. Hope this helps you too.

CreepColony's Opinion: The computer AI often brings ghosts along when they attack, mainly for lockdown. Does require some micromangement though.