Hi! I'm writing about the Battle Armor upgrades. The only place we can get them is the Strong Approaching challenge. But normal and even difficult only drop Common Threads. There isn't a way to get Medium and Advanced, only the shop, which will make it disproportionately more expensive to upgrade. I think you should match the drops to the difficulty levels. Easy should drop Common, Normal should drop Medium, and Hard should drop Advanced. It will still take a LOT of time to upgrade them this way.