The Archive Score & Incursion vs Meltdown

First off does anyone know what determines your score in story missions? Meaning what gives you a Bronze, Silver, or Gold rating at the end. My friend and I have beat every story mission running with just the two of us. We managed to get silver in every mission and even a few golds… all but the Archive. We have beat it 3 times now, all three times result in a bronze.

We have tried running the map and opening every single chest, including the little watermelon looking things growing on the trees. We activated every turret possible and got the items locked behind the purple barriers as well. Neither of us died or even went down once in any of the runs, the spider tank took almost zero damage through the entire mission. We also made sure to collect all drops from enemies including the boss. We both activated all our gear and had plenty of money left over after turrets as well. For the life of me I can not figure out what else we can do here?

Secondly this is just a side note… I really started thinking that I wish Incursion was the 2 lane style, and Meltdown was a single lane.

Bump for response please?