As for the kills. As someone else said, I personally would stalk my victim. Look for a spot I can shoot an arrow from sneak and not be seen, sometimes even in the same room as several people, then get out quick. If I feel more daring, then I will attempt to get the dagger sneak kill animation. And as for armour, I did prefer the nightingale chest stats to shrouded, but the same set bonus was better, also the stats on ancient shrouded armour meant my assassin Bosmer only wore the DB set as I was not putting points into smithing or enchantment. The bonus to sneak attacks and arhcery damage to a character that uses both were insane for me. As for the armour rating, you cant really expect to be a stealth killing machine, or if using some archery expect not to be weak when someone gets up close, the idea is to move, slow them down, kill them. Honestly level 50+ my assassin can not stand there and straight fight many enemies, but this is fine as I will switch to bow and create some distance. Same applies for mage, make distance and punish.
This is why I stopped playing mage and went Warrior, quite fun to actually get into peoples faces and fight.

