The game would be a lot better if the AI was better, more often than not it'll rush to take the one planet you can both get to at the start as quickly as it can, which means if you move second, you crush their forces that are weakened from taking out the shield, and then you can swiftly travel down the weakened line to kill them all off. If you want to conquer all planets before clearing the stage it only becomes a bit longer of a waiting game.