This is not terribly easy. Expect to retry levels a lot, and take frequent stress breaks to avoid throwing your Win Phone out the nearest window. Trust me on this.
That said, I found some videos of the iPad/iPhone version. The game is nearly identical on the Win Phone, so the vids are a great source of tactics, learning what to build, when, and where.
HTTP://www.youtube.com/view_play_list?p=2A8868339F854E64
The videos aren't perfect; there are a few where the player fails to "wipe them out". One tactic I used to get through is to create two paths from the enemy's start to the base. I then block path 'A' with a single soldier. Enemies will travel down path 'B'. If I run into a situation where they aren't being killed by the end of path B (the armored AT-STs are the worst for this), I sell off the soldier blocking A and buy one to block B. This forces the enemy to backtrack across path B and go down A. It was not uncommon for me to send the armored AT-STs back and forth five or six times before they would finally fall.