You could give wind walk the Slow Aura (tornado) buff so it doesn't have a buff and then make a trigger to every 0.20 seconds add your buff to your unit if it doesn't have it currently. That same trigger before adding the buff checks if the unit is doing the wind walk spell and if it doesn't it...
First of all, start all triggered spells with the event
A unit starts the effects of an ability
Next, the first ability is a simple
Pick every unit matching (matching unit is a hero equal to true < boolean condition) and do action - Set life of (picked unit) to 50%
Then fix the leak Custom...