Her survivability is based on the FEAR factor of you on the other team, ie: OH S*** WHERE DID SHE COME FROM, BAIL!!!!! or jumping in after tank is focused and kill everything in 5 seconds.
If you look at her skills & stats she isnt meant to be focused.... and spike/ravage destroys people fast, shadow walk for escape/stun. and an ulti that speeds you up when activated.
Survivability comes down to how you play and smart use of shadow walk and its cd. After all, if all the enemy champions are dead who's gonna kill you,
are you the assassin-(AP) that targets lone people (kills quick and vanishes doesnt need any hp to survive),
the unlucky jack of all trades-(AP&HP) who somehow finds himself often in 3-1 odds(ganks gone wrong or other team moves in groups thus need some hp),
or the feeder-(HP) who has been ganked all game and just wants to stop dying(been ganked hardcore to stop potential late game, also people who use oracle).
Item wise i dont have a set build, it changes depending on who i'm playing and their style but usually i go one 300 or more HP+ item, banshee veil to stop stuns and then mass AP. that gives a good balance of survivability and dmg.
Hope this helps and sorry if this doesn't make much sense, have not slept in a while.