Does Night Chill’s -cold resistance stack with Aura of Censure’s -elemental %?
Also, does -cold and -elemental % improve Frostburn damage? I know how this game treats Cold and Frostburn separately (weird).
If I have an item with +Elemental %, it will improve all 3 elements right?
Yes to all three.
One more question.
How do I farm lower level Uniques like Shards of Asterkarn?
kill a lot of enemy bosses/heroes that around the level of the item and pray rnjesus is on your side, being on a higher difficulty also helps
But on elite level, I level up so fast. LOL I feel like by the time I farm that item, I am already in ultimate. 
The drop rate for unique in Elite is way too low IMO.
Don’t bother farming on elite. Just clear the game with what you got. Once you’ve beaten the game and got as strong as you could along the way, see what works best for you in crucible (what waves at what difficulty) and farm gear from there. Or roguelike dungeons/wait for SR if you’re a crucible hater. There’s no shortcut to getting the exact regular uniques you need so maximizing unique drops per hour played is the only way.
Or trade.
Or GDStash.
Very hard to farm for low level legendaries (no idea what and unique item is) since 1) low drop rates 2) you spend less time in elite than in ultimate in general 3) few low level legandaries are as useful as their high-level counterpart so it does not make sense to farm low level legendaries
(Cruci drop rates need to get nerfed, seriously you can take a yellow item ravenous earth build and farm aspirant for a much higher leg/min rate than what any fully decked build can do in main campaign:P )