I'm sorry but how the heck are you doing that? The only thing that comes to mind is rare/reinforced chests for Lua/Void/Zariman, but they aren't guaranteed spawns so you have to account multiple runs of missions to 1 forma. Plaguestar having built forma is what makes it such an enticing event, so if you have a way to beat it, please share so that we can point out the DE that it IS undervalued.