Passive Enhancement Core farm from blue armor drops.
Since the drop is at 5 energy there is already 3 cores invested into it from the get go. What you want to do is level it up to 8 energy. This costs 3 cores, 1 prism, 10 legendary shards, and about 6,500 glimmer to do. And in return you get 6 cores back, doubling the cores you spent on leveling it up. Now it is very impractical to do as an actual farming method.
The best way to do it is when you get a random blue drop from an engram in the wild or after you get some blues from a playlist activity completion. You also want the ghost mod that gives you increased glimmer gains too.
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