The Campaign mission themselves are pretty hard (at least in my experience).
If you want to make things easier (though less enjoyable), you can use a cheat code to win every single battle in every Campaign scenario.
You simply press 'enter' during a game and type the following text: 'home run'.
This will ensure a swift and easy victory for you. No frustrating battles and hours upon hours of gameplay. Every single achievement regarding the Campaign achievements will unlock.
I hope this helps for the people who just want to play skirmish or multiplayer and want to have the Campaign done as soon as possible.
It's time for good old Age of Empires, have fun people!