Sorry that I'll be speaking broken English.
Hope that you understand what I'm saying.
Shadow Fiend is a imbalance hero to all others hero.
Because he have a high damage and combine with his armor reduction, it is a quite good hero.
Shadow Fiend is a good mid handle hero too.
Anyway , If SF fails mid, it is hardly to fat, because when he failed ,it will be 80% about losing the game, 20% to win.
If he get well, I guess the game will win easily.
Firstly , I will introduce, what Item I'm going to buy or use.
If you are going to mid lane, you can buy Wraith band + 2 branches , in case , you can call your teammate to lend you tango too.
If your mid is steal by others, nevermind , you can go to bot or top lane, whatever you like.You can buy yourself a tango, 3 branches , and 1 magic stick. Because maybe you will meet some skillers.
In 10 minutes, you should have a Power Treads, RoQ,Magic Wand and a bottle (Highly Recommended), or you don't want the bottle also can.
Safely gank, ok, you can buy shadow blade with dagger, you know why?
Shadow Blade combo with ulti, and dagger is combo with your damage or your skill too,
When you meet some problem you can use Shadow Blade to run away and use your dagger blink away too , so that enemies will hard to chase you.
After you have a safe gank Shadow blade or Dagger,
now are the main core items time,
you may build Daedalus, Manta, Desolator, MkB( For Those skill Miss hero, Highly Recommended)
Other cases, you can buy Eye of skadi too or VoS (Hex)
Oh Yes I almost forget , you must buy bkb to get your safe, otherwise you will gank by those skillers.So that you can buy your next main core items.
In Fact, there's my style , without bkb ( not recommended you play if you think it was a noob) , Get Shadow Blade or dagger, and Power Treads or BoT, after that you should get a Mask of Madness , Basher, Crystalys,Maelstorm when you are in the middle game, later after you are going to the late game, you may have a daedalus , Abyssal blade and Mjolnir
, or get a MKB if you meet those skill miss or miss item
Other cases, you can buy Santanic combo with you Daedalus, high damage with high lifesteal. With Mjolnir high Speed too.
If you want to play tanker style , you can buy HoT, Assault Cuirass , Eye of Skadi ( slow the person) or hex. That's it.
With High attack speed
you can buy assault Cuirass( you can combo with your skill 3, and reduce more physical resistance) , Manta , Mjolnir, Butterfly
My normal style is
BoT, Butterfly, Manta , Daedalus, Desolator, BKB, Monkey King Bar.
if you want to team work ,
you teammate may choose Faceless Void (ulti), Tide ( slow and ulti) , Magnus ( ulti and his drag), Es is a good hero too with his stun.
Any hero are can combo with sf,
it is base on the player how to play the sf and how to control sf well.
I want to say that is
If you meet a skiller team, sf is better to hide, because if you are unfortunately stunned or slowed, you maybe die at the next second.
SF cons is meeting those skillers with a gang.
Anybody hates stunned or slowed, but SF hates it well.
With a good highly damage skill
With a good ulti
With a good armor reduction
With a good high damage
With a good attacking speed