I am quite certain that success in Defence missions is NOT based on how full your health bar is at the end, but instead it's based on HOW FEW HITS IN TOTAL you took. I'm saying this because I finished Defence 1 with full health (!!) at the end of the mission through judicious use of the Energy Cell - Azote Chilling System (less cooldown) combo and collecting Medkits at the end of the level and I got a single bronze star. I then tried Defence 1 again with Time Control Device 2.0 - Azote Chilling System and I did my best to minimize the number of hits I took in total, finished with 305/310 health, and I got silver instead of bronze!
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