I will help you on this. It is a must to have
Overdraw(5/5), Critical Shot(3/3), and
Power Shot. I also recommend
Eagle Eye andat least one point in
Steady Hand. If you want to be sneaky (I am always sneaky), the entire
Sneak tree is what I recommend, but if it's not your cup of tea then you must use,
Stealth(5/5), Deadly Aim, and I also recommend at least
Muffled Movement. As I think about it,
Backstab is kind of handy as well. The
Conjuration tree is tricky. It depends on what kind on conjurer you are. I don't recommend a lot of HP for sneaky magic-using archers. You don't need high HP as you are sniping at long range. There should be no close combat. Put points into
Magicka. Also sneaky characters need to be strong in the
Illusion tree (the whole tree). Do this and "One shot, one kill," is your mantra.

Oh yeah, before I forget spells. I highly recommend
Flaming Familiar as an apex summoning spell. It is a quest given spell, but if you haven't found it in other playthroughs just keep hitting up the draugr tombs and you will find it.
Always travel with a companion outside, but make them wait while you are inside, or if you are assasinating a specific mark.

Cheers!