I think it's just getting a specific amount of points per level. Like above 2 million points on hard for most of the levels. Most of the time it is essential to kill those invisible ships. Acquiring high combos and sustaining the multipliers is also highly recommended.
I'm only missing two levels to 5-star on hard, these are: "Asteroids" Mission 6 and "Fire" Mission 9... most of the time I have a hard time surviving those levels. "Ice" Missions where a piece of cake... Those two above mentioned are also hard to get on normal, I think.
Maybe it's easier to achieve those goals on multiplayer?
Last edited by Kaipiranha; 09-27-2008 at 11:24 AM.