From StrategyWiki, the free strategy guide and walkthrough wiki

Right now, most of the missions do not have a walkthrough--just a description of each mission. The actual walkthroughs will begin after the mission details are done.

Also, please note that the mission difficulties represent how hard the mission is the first time it is played. Most missions become substantially easier once a player learns how to beat it.

Before you read a walkthrough for a mission, make sure you

• have tried the mission at least two times on your own, without help
• are not reading ahead (for example, you're playing the first mission but reading the walkthroughs for all missions). This really messes up your game, and you'll be getting plot spoilers all over the place, albeit with warnings.