You have to get 5 stars on speed in the last level in Advanced Campaign.
If you can´t do it, try to rush with Lareina, because her power is pretty useful against the AI. If you haven´t used it, it makes every unit near you invincible for the next turn. It´s a really good way to advance when you are on a tight position, or when you need to know what the enemy have.
But most importantly, the AI doesn´t recognize when you are invincible, so you will strike two times: In your turn, where you´ll be able to advance and attack any units you can without any risk, and in his turn, as he will attack you and damage himself in the process.
If you are in danger, you can always retreat again to a safe position, after knowing which units do the AI have, and making some serious damage. Don´t forget that invincible infantry could take the enemy´s properties, even being surrounded. Use it that to your advantage too.