Played this game years ago. Good to come back to it. RPG elements and numerous upgrades to rifle's spread, ROF, and damage means your character gets massively overpowered by the last level. Still attempting to get four stars in each level was a frustrating but very fun challenge...until I got to the Bridge. I thought a no-damage clear of the level would guarantee all four stars. In levels 1 through 3, it DID.
For The Bridge, I did a no-damage clear TWICE and got only 1 star both times. I don't know what's going on there. There needs to be some indication of what it takes to get 4 stars.
Update for all the 0 people that care: It's not clearing the mission with no damage. It's clearing the mission as fast as possible.