How to murder efficiently as an assassin?
A sneak build cuts though 95% of the game like butter, but it requires some adjustment for the other 5%: dragons, surprise outdoor encounters, bosses that take you out of stealth for a dialogue when you enter their rooms, and some melee bosses with high armor and health and a location that makes it hard to sneak away in between attacks.
Pick up a horse and a heavy armor follower, and consider putting points into conjuration, to provide lots of tankage when you need it. Make sure to keep your follower tank in good gear (in the latest version they now level up along with you.) In dungeons you'll want to leave your follower behind and go get them when you've cleared up to a boss, because the AI stinks for stealth support.
Invest in a bit of stamina, wear light armor, and kite like crazy (sprint away and then shoot, using the unrelenting force shout to help keep enemies at a distance). This gets much stronger once you pick up stagger in the archery tree and max out unrelenting force.
Get the 3x bow and 15x 1h damage perks in the stealth tree as soon as possible.
Put points into smithing and stay at least a bit ahead of the gear curve, and do at least a little enchanting (+fire/frost weapon enchants in particular seem very strong out of the gate) and alchemy to stock up on poisons for your arrows. Because of this, right now I'm stealth one-shotting everything in dungeons but bosses. This lets me save up potions and scrolls for those few tough encounters.
Look into going down the illusion path for muffle and invisibility. I haven't done this personally since you don't want to spread yourself too thin, it doesn't help with dragons like conjuration, and I like the occasional big messy surprise fight, but I hear it works well also.
Usually when playing an assassin build, you need to be able to either kill or take a large chunk of health off of an enemy before they notice you, or get close to you.
In my stealth archer build, I've got a number of perks invested into archery and a number of bows do % more damage armour pieces. I may be level 36~, but I can do about 450 damage in a sneak shot.
Also, use poisons. The extra damage is really useful, although they do not benefit from sneak multipliers
If you meet a group of enemies, spells like Frenzy can be very useful.
Getting a companion or something similar (conjuration?) can also be good to tank for you. If you're sneaking, your follower won't ruin it by running in. I currently have Lydia + the Sanguine Rose (summons a Daedra to tank for you)
Make sure to stay undetected after the first hit, if it doesn't kill. That usually means being far away from the target, which can be hard for random encounters in the wild (or dungeons with lots of bends).
Also, if you find an enemy that you can't take down with a single hit, improve your chances: use poison, use the best arrows that you carry around (daedric arrows are great, but very rare) and if at all possible: improve (smithing) and enchant (enchanting) your bow for a better chance of one-hit kills.