Give a custom-enchanted dwarven item to Calcelmo for his quest

You cannot replace enchantments, nor can you add to already enchanted items with the Extra Effect perk.

It is better to wait until you have a high Enchanting skill and the Extra Effect perk before enchanting good equipment, unless you have the materials to make another one.

Unfortunately, sneaky one shots are people using daggers (I use war axes from time to time as well) for the most part, not bows.

However, assuming the confirmed knowledge that you will never be able to drop all your targets in one hit has not stopped you from wanting to pursue an archer here are some pointers to consider.

• Sneak attacks are your openers.
• Just like an assassin doing a backstab, picking your initial location to start an engagement is key.
• Being able to slow your target is going to give you that much more time to deal with them. Consider the poisons that Eriuzo mentioned but more so, consider enchanting your bow with the highest charge lowest duration Frost you can get. With fast draw and good aim (which will get better the more you use it), you should be constantly pelting your target.
• Also consider the Paralyze enchantment in similar use to the frost above, you might want to fiddle with charges vs duration a bit more with this one however.
• Still go max sneak perks, as you will want to often be at range the entire time and being able to drop into a sneak to break targeting will be very helpful.
• Remember that Mobility is going to be very key for you, do not neglect stamina!
• If you find running around and shooting too tedious and just want to stare them down with your bow, remember you can block/bash with your bow.. Consider getting some of those perks to make your shoves matter more.
• Just as the mages' Impact perk is important for staggering, so too is the archery 50/50 stagger perk.
• Seek out +Bow gear as soon as you can, every little bit helps. (There is some in the Dark Brotherhood and its fairly short to get to, unlike the horse which seemed to take forever to get the second time around :))
• Consider high perked pick pocketing.. The best arrows in the game are fairly annoying to come by. Some of the ways to generate these arrows better are to go to places where people practice archery and steal all their arrows, especially their active one, and then give them a single Daedric arrow.. They will then shoot out an infinite number of these that you can simply pick up off the target. Note that I am still torn about this as this is -a lot- of perk points just to get arrows... Id almost say go to the fletcher, buy the arrows, save the game, kill the fletcher, reload the game and start the loop over as a more 'acceptable' way to overcome this.

Because possible enchantments on your weapon can play a pretty big part of how your character does what it does, consider doing the quest for Azura's star and turning it into the Black Star. Right now it can accept any soul and if you are taking out a bandit camp you can recharge every weapon fairly quickly with a grand soul they give you.

EDIT: Technically, there is a way within the game rules to make a bow powerful enough to one shot anything... but it is rather game breaking if you abuse the loop too much. Through Alchemy and Enchanting you can boost your smithing skill in an infinite loop. People have made weapons and armor with stats so high that it flips the internal value into the negatives (so, higher than 2.1 billion).

The technique to do this is commonly called the Restoration Loop