It can drop from 750, and no, score and time do not affect chest loot, where it comes from. In fact, score & time do not affect anything. Only the rank matters, which only affects ordeal rewards like upgrade materials. So in the case of farming nightfall exclusives like Osprey, even that doesn't matter. (Edit: 750 is always platinum anyway) Your most efficient method would be to speed through the 750 as fast as possible, focusing only on completions.