I played this level several different times, and here is my first highscore guide for the PlainPlane level in the First Encounters campaign:
First, place a Plasmatron on the right side of the exit of the Alien Spawns, then, place a row of about three Lasers and a Mortar to the left of the Plasmatron, as shown:
Then, hold down the send waves button for a couple of seconds and it will send all waves.
Upgrade the towers in the front, then add another row of Lasers behind the front row, like so:
Now you have your base, all you need to do is keep upgrading both rows of towers, and, if you want you can but some more towers back by your base as backup.
As long as you are fast to upgrade your towers when you get money, you should defeat every wave and keep your sheep safe, and also get highscore by sending all waves early and killing all enemies.