Originally Posted by Wafe
I could be wrong, but this SHOULD cast Wind Shear on your target's target if you have shift down, and Purge otherwise. If your target's target isn't hostile it'll cast Purge at your target instead.
/cast [mod:shift, 'AT'targettarget,harm] Wind Shear; ['AT'targettarget,harm]  Purge;
The issue with your original macro was that you used [ ] brackets twice before the first spell, what this does is create two separate ways to trigger that one spell.
Again, using 'AT' instead of the symbol due to low post count.