CC her then focus, nothing she can do about that if you are all outside her Ults range. This is the biggest way to counter her, stopping her while you are outside her ult range (which is decently small) and she will just stand there and die.
Wait for her to Q once then hard CC her, it will make sure she cant Q again.
Run away from her, she doesnt have a MS steroid without hitting something so if her Q is down or you are out of range then she is just a useless pile of health that can be kited.
Warding and good Map Awareness stop her from ganking you often.
Kind of weird but going invisible doesn't stop her ult but it does stop her from dealing damage to you.
She can still be effected by DoT's, AoE's, and any damage that got off before she ulted. I have been killed by old Wraith camp before while in my ult simply because their AA's went off before i ulted and hit me after i ulted.
Armor, specifically Thornmail, she tends to build Armor Pen so you need a lot to make a difference. Thornmail is really good against her, she is squishy so any extra damage she takes is helpful.
When her W (the parry) is up dont AA her.