Hi everyone , This my own experience playing with Crystal m as a midder (I love her so much! I'm crazy about her) and without any further ado let's jump to the guide.
So the items I ussally purchase are one iron brach , one faerie fire , a mango , a ward , the courier , and finally the mantle of intelligence. you can always use dota plus help for the late game items and build.
you wouldn't try this vs blood seeker . He has high speed attack at the beggining you won't be able to get enough denies ( you don't need as much last hits and denies as a carry hero because you're level will get higher sooner than your enemy. I mean you need less denies and last hits than the other carries to level up )
The point of being good with Crystal m mid is to kill the enemy at the beggining as you get you fourth ability so their midder won't grow.So if your enemy can escape or is a strengthen hero you probably shouldn't go mid because they won't die under you freezing feild.
Though for the heroes like sniper or meepo you can easily get them with your freezing field and their midder won't grow enough.
The other thing is your team needs to push and finish fast because in the late game your enemy will get the chance to win.
That's because you were mid and you need a really good team to fight and if you let your enemy to the late game they could get stronger and since you are the midder and your team has lower xp you can't do much.
Sorry for my bad english. Imma add more stuff and things soon. Sorry for my bad english by the way.
Well that's it I hope it helps enjoy!

Bye !