Here are the videos that helped me for Scrin missions.
Mission 4-Threshold 19
Note for Mission 3- The tactic used in Mission 3 in the video is very difficult to pull off, but is doable. It just needs to be done a little differently than in the video, since engineers on 360 take longer to capture a building than on pc.
For starters, teleport your two acid bugs to the far back right GDI power core, and then attack it through the fence. Your aim here is to disable power to the GDI base so you can capture the Construction Yard and War Factory without being instantly shot by the sentry turrets.
Another difference from the video is that you will need a lot of anti vehicle turrets and anti-vehicle infantry, because Nod will come at you hard as hell after you neutralize the GDI war factory.
Now you need to teleport into the Nod base as shown in the video, and take the construction yard, followed by a couple of power plants to take out their defenses. After they are powered down, either take some more power plants or take out their war factories. Once you've done this you can clear up the GDI base for the bonus objective.
Then when everything is secured, teleport some acid bugs to destroy their back 3 obelisks of light, and then teleport some engineers in to capture the 2 buildings.
The other 3 missions are pretty straightforward, only Mission 3 requires an unorthodox tactic.