Caltrops (Hot Pursuit can work) -> Irrelevant -> Any/Build Synergy -> Rain of Vengeance -> Tempered by Discipline (not required) -> Seething Hatred or Frost Shot -> Rancor or Storm of Vengeance.
The reason Valla has an advantage is that Illidan cannot reset his abilities off of her passive if he eats a slow before a second melee. Caltrops breaks him, and all she has to do is a semi-decent Vault to start the kiting chain. Once he’s locked out of both Dive and Sweeping Strike, he’s completely helpless to Rain of Vengeance unless he has Metamorphosis. However, Valla can be built to still have the advantage here, since she can reset the distance and have Vault back up for Caltrops once again.
TL;DR: Valla can build and outskill Illidan, and there isn’t much he can do about it except hope she makes a mistake.